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.