Republicans were all but assured of winning Democratic-held seats in West Virginia, Montana and South Dakota, and Democrats held out little hope that Sen. Mark Pryor could win re-election in Arkansas.
Polls suggested that races for Democratic-held seats in Iowa, Colorado and Alaska have tilted the Republicans' way, too—although Democrats said their get-out-the-vote operation made any predictions unreliable.
Democratic incumbents faced competitive races in New Hampshire and in North Carolina where Democrats said they had an edge—and Republicans disagreed.
North Carolina Sen. Kay Hagan was one Democrat in a marquee race quietly accepting a bit of last-minute help from the president. She has spent much of the year distancing herself from Obama, but her campaign sponsored a radio ad featuring Obama calling her a tireless leader “who shares our priorities.”
It was unclear where Hagan's campaign was airing the ads, but other candidates have used similar ads to boost turnout among African-American voters still loyal to the president.
Strategists in both parties said candidates in Louisiana and Georgia were unlikely to reach the 50-percent threshold needed to avoid a runoff. The wildest wild card of all was in Kansas, where polls said 78-year-old Republican Sen. Pat Roberts was in a close race with independent Greg Orman in a state that has only sent Republicans to the Senate for nearly 80 years.
Democrats had hoped to pick up the Kentucky seat held by Senate Republican leader Mitch McConnell, but recent polls showed him building a lead over Democratic challenger Alison Lundergan Grimes. McConnell would be in line to control the Senate's agenda as majority leader if Republicans win on Tuesday
That left Georgia as the Democrats' best opportunity to pick up a Republican seat, with Democrat Michelle Nunn, whose father served four six-year terms in the Senate, facing Republican businessman David Perdue.
Partisan fighting and legislative inaction have already characterized Washington in recent years, peaking in 2013 with budget fights that shut down the government and raised the specter of a default on the federal debt.
With a Democrat in the White House and Republicans in control of both chambers of Congress, a big question is whether the legislative paralysis would deepen or whether political reality will push both sides to compromise at least on modest goals.
Republicans may be emboldened to intensify conflicts with an unpopular president in the last years of his term, but they risk Obama's veto if they push too hard for lower taxes, fewer regulations and other conservative priorities. Trade agreements that Obama supports, and changes to the immigration system that many Republicans favor, could offer chances for agreements.
Obama was back at the White House after making his final campaign appearances over the weekend. He raised tens of millions of dollars over two years for Democratic candidates, but in the campaign's final days he campaigned mostly for Democratic gubernatorial candidates in states he carried in 2008 and 2012.
Also on the ballot were gubernatorial elections in 36 states, and an unusual number of incumbents from both parties appeared to be struggling.
Among the most closely watched is Wisconsin, where Republican Gov. Scott Walker is in neck-and-neck race with Democratic challenger Mary Burke. Walker, a favorite of conservative Republicans, is often mentioned as a potential candidate in 2016, but his White House chances would almost certainly evaporate if he lost Tuesday.
In another hard-fought race, Florida Republican Gov. Rick Scott is facing a tough challenge from Charlie Crist, a former Republican governor-turned-Democrat.
Early voting topped 18 million ballots in 32 states, and both parties seized on the number as evidence of their own strength.
