Democrats appear primed to pick up some seats in the Senate, but a number of other hotly contested races are tightening up, increasing the chances that Republicans may be able to retain their majority.
Democrats need a gain of six seats to win back the Senate, which they last controlled in 2002. Republicans now control 55 seats in the 100-member chamber.
Among the races to watch:
Two other Republican senators, George Allen of Virginia and Jim Talent of Missouri, are locked in dead heats with their Democratic opponents, while GOP Sens. Rick Santorum and Sen. Mike DeWine of Pennsylvania and Ohio, respectively, trail their Democratic challengers and are considered in serious trouble.
Among Democratic incumbents, only Sen. Robert Menendez of New Jersey was considered in any danger. He held a 48-43 percent edge over Republican challenger Tom Kean Jr. in a new Quinnipiac University poll. Other recent polls have given the Democrat as much as a 10-point lead.
On the House side, Democrats appear poised to regain control after 12 years in a shift that likely would elevate Democratic Rep. Nancy Pelosi of California to speaker, the nation's first woman to hold that office, and herald in at least two years of Democratic rule.
Pelosi cautiously optimistic about her party's chances. "We are thankful for where we are today, to be poised for success," she said in Colchester, Conn. "But we have two Mount Everests we have to climb — they are called Monday and Tuesday."
At least 50 Republican seats are endangered, many with incumbents facing fierce challenges from Democrats who have sought to capitalize on the public's intense disenchantment with one-party rule.
Republicans are hoping their acclaimed get-out-the-vote operation will ensure majority control. But some say privately they have a slim chance of retaining the House after a grueling campaign centered on turmoil in Iraq, President Bush's sagging approval numbers, political scandals and corruption investigations.
"It's hard to see a scenario where the Democrats don't pick up the House," Joe Lockhart, former press secretary for President Bill Clinton, told Early Show co-anchor Hannah Storm Monday. "I think Democrats are privately worried that their supporters have seen this lead for so long now — weeks and weeks — and the people are going to become complacent."
Republican political consultant Nicole Wallace told The Early Show that despite the Republicans' problems with political scandals and the Iraq war's growing unpopularity, the GOP is "privately a little excited" that the midterm elections are so tight.
Republicans repeated their assertion that Democrats would raise taxes and prematurely pull out of Iraq if they controlled Congress. Democrats pressed their case for change, arguing that Republicans on Capitol Hill blindly have followed President Bush's "failed policy."
Iraq has dominated the campaign season, and Republicans and Democrats sparred over the war again Sunday following Saddam Hussein's conviction on crimes against humanity. He was sentenced to die by hanging; an appeal is planned.
President Bush was spending Monday urging Republicans in Southern states to get out and vote.
Despite the Republicans' urgency, Mr. Bush is campaigning in areas that are very, very safe — places where protesters will most likely be absent, reports CBS News senior White House correspondent Bill Plante from Crawford, Texas.
"Here's the way I see it," the president told a crowd Sunday in Grand Island, Neb. "If the Democrats are so good about being the party of the opposition, let's just keep them in the opposition."
Also up for grabs Tuesday are governorships in 36 states, and thousands of state legislative and local races. In 37 states, voters also will determine the fate of ballot initiatives, including whether to ban gay marriage, raise the minimum wage, endorse expanded embryonic stem cell research and — in South Dakota — impose the country's most stringent abortion restrictions.