Tonight, after the polls have closed and votes have been tallied, Americans will have elected the next president of the United States. Regardless of whether the victory goes to President Obama or Mitt Romney, however, the next president's ability to govern will hinge on which party holds a majority in the House and the Senate.
In order to overtake the slim Democratic majority in the Senate, Republicans need to pick up at least four seats tonight. A year ago, that seemed like a pretty reasonable goal for the GOP. But in the final days of the campaign season, amid a handful of unforced errors by Republican Senate candidates, it's looked increasingly out of reach. Democrats, meanwhile, seem equally unlikely to pick up the 25 seats they'd need to regain control of the House.
Below, CBS News takes a look at the top House and Senate races to watch as polls close across the country.
7:00 p.m. ET
Indiana Senate: The Indiana Senate race has been full of surprises for Republicans: First, conservative Richard Mourdock defeated longtime moderate incumbent Dick Lugar in primary; more recently, after Mourdockthat pregnancies resulting from rapes were "something that God intended to happen," an already-close race tilted in conservative Democrat Joe Donnelly's direction. A Donnelly win there tonight would make it even tougher for Republicans to overtake Democrats in the chamber.
Virginia Senate: In Virginia, voters face a familiar pair of candidates when making their choice for the Senate: Tim Kaine, the Democratic former governor, is running against Republican George Allen, a former senator and governor, for the state's open seat. The contest has been among the most closely-watched in the country, particularly given Virginia's status as a critical swing state in the presidential election, and was neck-and-neck for months. In recent weeks, however, Kaine appears to have eked out a slim lead.
7:30 p.m. ET
Ohio Senate: Democratic Sen. Sherrod Brown started his re-election campaign with a solid lead over his competitor, Ohio Treasurer Josh Mandel. But amid a recent onslaught of outside money, his lead appears to have winnowed down to just a few points, giving Mandel some cause for hope. If he's successful in unseating Brown, Mandel would be handing the GOP a much-needed pick-up: Even if Republicans can win seats in all the states where Republican Senate candidates are either tied or have the lead, they'll still be one Senator short of a tie with Democrats without a victory in Ohio, according to the New York Times.
Ohio - U.S. House, 16th District: GOP Rep Jim Renacci v. Democratic Rep. Betty Sutton: Because Ohio lost two congressional seats after the 2010 census, Renacci and Sutton are forced to run against each other. The bitterly fought campaign between the two candidates has also attracted millions of dollars from outside groups who are waging a vocal war in the race.
North Carolina - U.S. House, 7th District : Democratic Rep. Mike McIntyre v. David Rouzer: Incumbent Rep. Mike McIntyre is one of the endangered conservative Blue Dog Democrats who represent a district that leans Republican. His newly mapped district includes suburbs of Raleigh and areas along the coast. His challenger, state senator David Rouzer has received the backing of outside groups in a race that Republicans hope help dash Democrats' hopes of picking up seats. But because he survived the Republican landslide in 2010, when the GOP picked up more than 60 House seats, McIntyre is a viable candidate.
8:00 p.m. ET
Connecticut Senate: Despite her loss to Democrat Richard Blumenthal in 2010, Linda McMahon, the co-founder of World Wrestling Entertainment Inc., was not deterred from another Senate bid - this time to replace outgoing Sen. Joseph Lieberman, an independent who caucuses with Democrats. This year, after revamping her controversial image and adopting a more moderate tone (in a recent mailing, she encouraged voters to vote a split ticket for her and President Obama), McMahon appeared to be mounting an unexpected challenge against Democratic Rep. Christopher Murphy. Recent polls, however, show Murphy gaining steam, making a Republican pick-up in the state less likely.
Florida - U.S. House, 18th District: Republican Rep. Allen West vs. Patrick Murphy: Tea Party favorite Rep. Allen West swept into Congress with the Republican landslide in 2010. Redistricting has forced him to fight for reelection in a new South Florida district, which may wind up being a fortuitous turn of events since the electorate is slightly more Republican than his current district. His $18 million fundraising haul might also play a large role should he be reelected against 29-year old Republican-turned-Democrat Patrick Murphy. The race has been ugly with one ad depicting West as a gold-toothed thug and another posting Murphy's mug shot from a college bar brawl.
Illinois - U.S. House 8th District: Republican Rep. Joe Walsh vs. Democrat Tammy Duckworth: Walsh, the Tea Party backed Freshman is facing double amputee Iraq War vet Tammy Duckworth in the newly redrawn and more Democratic district. Walsh has been one of the several Republicans under fire for his position on abortion because of his opposition to exemptions of rape and incest. He also said recently that "you can't find one instance" where abortion would save a woman's life or protect her health.
Maine Senate: In Maine, the independent former governor Angus King is running to replace outgoing Sen. Olympia Snowe - a longtime moderate Republican who announced her retirement earlier this year. The seat is seen as a pick-up for Democrats, although King has vowed that, if elected, he would take "the best ideas, regardless of party, to find common sense solutions" in the Senate. Currently, he appears to have a hefty lead in the polls over Republican Charlie Summers and Democrat Cynthia Dill.
Massachusetts Senate: Perhaps the highest-profile Senate race is taking place this year in a state where the presidential election is hardly being contested: Massachusetts, a reliably Democratic state, is a safe bet for President Obama even despite the fact that Mitt Romney once served as its governor. Between Democratic candidate Elizabeth Warren and incumbent Republican Senator Scott Brown, however, the outcome is far less certain: While Warren, a liberal icon and consumer advocate, appeared to have eked out an edge in recent weeks, at least one new poll shows Brown in the lead. Both candidates boast at least one inherent edge: Warren will benefit from the president's support, while Brown has the advantage of the incumbency.
Massachusetts - U.S. House, 6th District: Democratic Rep. John Tierney v. Richard Tisei: Democratic incumbent Rep. John Tierney has represented his district in the House since 1997 and is facing defeat in large part due to his wife's family illegal gambling business. Challenger Tisei is a fiscal conservative and would be the first openly gay Republican elected to Congress.
Missouri Senate: Until August of this year, Claire McCaskill, Missouri's incumbent Democratic senator, was considered one of the most vulnerable Senate Democrats up for re-election. Elected by a razor-thin edge in 2006 in a state that has since grown increasingly conservative, Republicans were counting on picking up a Senate seat in Missouri. But after a protracted primary fight yielded the ultra-conservative Rep. Todd Akin as the Republican nominee, the GOP has faced some set-backs in the state. Chief among those was Akin's August comments suggesting, inaccurately, that pregnancies don't result from "legitimate" rapes. Since then, McCaskill appears to be leading in the polls, and Akin has struggled to stay competitive.
9:00 p.m. ET
Arizona Senate: The battle to replace outgoing Republican Senator Jon Kyl, Arizona's junior Senator, has ramped up in recent weeks, with former surgeon general Richard Carmona edging in on Republican Rep. Jeff Flake. Flake entered the race as a strong favorite, but a recent infusion of outside cash seems to have helped give Carmona a boost.
Arizona - U.S. House, 9th District: Democrat Krysten Sinema v. Republican Vernon Parker: In one of Arizona's new congressional seats that includes parts of Phoenix and Tempe, the race between Sinema, who is openly bisexual with a politically radical past, and Parker, an African-American up-and-coming Republican, is one of the most watched Arizona House races. Each candidate is labeling the other as extreme and attacking at every corner.
Colorado - U.S. House, 6th District: Republican Rep. Mike Coffman v. Joe Miklosi: Rep. Mike Coffman is the incumbent facing state representative Joe Miklosi in the suburban district that stretches from Denver to Aurora, Colo. The newly remapped district includes a larger percentage of Democrats. Coffman's Tea Party principles are being challenged as he's trying to focus the race away from social issues, including abortion, which continues to plague his candidacy.
Minnesota - U.S. House, 6th District: Republican Rep. Michele Bachmann v. Democrat Jim Graves: Tea Party star Michele Bachmann spent much of 2011 on the presidential campaign trail. Now she has a formidable challenger in Jim Graves. But her fundraising heft is incomparable. She has raised at least $20 million dollars and spent most of it trying to hold on to her seat, according to the Center for Responsive Politics. Graves is running as a moderate and framing Bachmann as divisive.
Nebraska Senate: After Nebraska's moderate Democratic Senator Ben Nelson announced last year he wouldn't seek re-election, his party's prospects of holding onto the seat looked bleak. Even Democrat Bob Kerrey, a former senator and governor and well-known political entity in the state, doesn't seem to have much hope of winning a state that is now solidly Republican. Despite some recent tightening of the race, Nebraska state senator Deb Fischer appears to have the lead there.
Texas Senate: Tonight's Texas Senate contest isn't expected to be a close one: The Lone Star State isn't a battleground territory (though President Obama predicts it will be soon), and Republican Ted Cruz is expected to defeat Democrat Paul Sadler handily. But Tuesday will mark the culminating point of Cruz's unexpected rise: The Tea Party-backed candidate has no history in elected office, and is being touted as one of the conservative movement's rising stars.
Wisconsin Senate: In Wisconsin, Democrat Tammy Baldwin is fighting tooth-and-nail to beat former Gov. Tommy Thompson for the state's Senate seat being vacated by retiring Sen. Herb Kohl, D-Wis. If she wins, Baldwin will be the first openly gay woman elected to the Senate. She'll also help Democrats keep the seat blue. But as with the presidential contest in Wisconsin, the race is exceptionally tight: A late-October poll from NBC/WSJ/Marist had Baldwin leading by just a point. As with other close Senate races in battleground states, the fate of the Wisconsin Senate could well hinge on which presidential candidate is best able to turn out his supporters.
10:00 p.m. ET
Iowa - U.S. House, 3rd District: Democratic Rep. Leonard Boswell v. Republican Rep. Tom Latham: Redistricting led to these two sitting members of Congress to run against each other. Latham, the Republican, has easily outraised Boswell who has never been considered a proficient campaigner. Outside groups, especially those backing Latham, including Karl Rove's group, have also played a major role in this race. Latham is also close friends with House Speaker John Boehner, a tie that has provided national support and funding.
Iowa - U.S. House, 4th District: Republican Rep. Steve King v. Christie Vilsack: This race pits two well-known names against each other. King is the outspoken Tea Party-backed conservative incumbent and his challenger, Vilsack, is the wife of former Iowa Governor and current Agriculture Secretary Tom Vilsack. The newly redrawn district is geographically large and leans Republican - good news for the conservative firebrand who might put off some moderate and independent voters.
Nevada Senate: Nevada's Senate race, between appointed Republican Sen. Dean Heller and Democratic Rep. Shelly Berkley, is a pick-up opportunity for Democrats. It's also a close race. Harry Reid's strong turnout machine in the state has been touted as critical to President Obama's apparent advantage there, but polls suggest that Berkley continues to trail her Republican rival. The possible good news for Berkley? Nevada polls are notoriously bad.
Utah - U.S. House, 4th District: Democratic Rep. Jim Matheson v. Mia Love: Six-term Congressman Jim Matheson is up against Mia Love, whom some consider a rising star in the Republican Party. She is an African-American Mormon and the mayor of Sarasota Springs, Utah. She would be the first black Republican woman elected to Congress. Romney's candidacy will help Love but Rep. Matheson is a moderate Democrat, a member of the fiscally conservative Blue Dog Democrats, and a popular Democrat whose father served as governor.
11:00 p.m. ET
California - U.S. House, 27th District: Democrat Rep. Howard Berman v. Democrat Rep. Brad Sherman: California's redistricting and jungle primary are the reasons these two members of Congress from the same party are running one of the fiercest contests in the country. Both are liberal Democrats and Jewish supporters of Israel, but their similarities didn't matter much during a debate last month where the two nearly came to blows. Members of Congress, both Republicans and Democrats have also taken sides.
An original version of this story incorrectly indicated that Rep. Sutton challenged Rep. Dennis Kucinich in a primary.