WASHINGTON — Democrats retained a narrow majority in the Senate on Tuesday, but Republicans kept their grip on the House, delivering another divided, and highly polarized, Congress.
The balance of power was likely to shift by no more than a seat or two, if at all. Neither record-low job approval ratings nor an avalanche of campaign spending appeared able to shake the dynamic that made the last Congress the most partisan since the Civil War.
"That's the sort of sad state of affairs: You're not going to have much change in Congress," said Keith Poole, a political science professor at the University of Georgia, who has researched decades of congressional voting patterns. "That's a real recipe for confrontation after the election."
Republicans had high hopes of wresting control of the Senate from Democrats, as President Obama's popularity slid and Democratic incumbents faced a less favorable political climate than six years ago, when many first-term senators won in a wave that gave their party the majority.
Democrats had nearly two dozen seats to defend, twice as many as Republicans, who needed four seats to tip the 53-47 balance — or three if Mitt Romney had become the Republican president and Rep. Paul D. Ryan the tie-breaking vote in the Senate.
But the decision by Republican leaders to stay out of the primary process ceded the field to tea party candidates who then struggled in key states. Voters rejected those conservative Republicans in Missouri and Indiana.
Republicans also lost their marquee race in Massachusetts, where Sen. Scott Brown helped launch the 2010 tea party wave by winning the seat that came open after Sen. Edward M. Kennedy died. He was defeated by Elizabeth Warren, the liberal Harvard professor who had been Obama's choice to run the new Consumer Financial Protection Bureau.
At the same time, Democratic candidates held their own in key swing states. In Ohio, one of the most liberal Democrats, Sen. Sherrod Brown, won a second term, while in Florida, Democratic Sen. Bill Nelson was easily reelected.
In Wisconsin, Democrat Tammy Baldwin, the congresswoman from Madison, prevailed over former Gov. Tommy Thompson to become the first openly gay senator.
However, Montana's Democratic Sen. Jon Tester faced a difficult challenge from Republican Rep. Denny Rehberg.
One critical battleground, Virginia, was a race between two political giants, Tim Kaine and George Allen, both former governors. Kaine, who made an appeal to women, minorities and independents put off by Allen's conservative tilt, captured the seat. Allen, the towering son of the former football coach, had sought to retake the Senate seat he lost six years ago after uttering a racial slur.
With the once-broad playing field narrowed, the chance for a wholesale makeover in the Senate, and with it a mandate for governing, slipped away from the Republicans.
Conservative Republican Senate candidates Todd Akin in Missouri and Richard Mourdock in Indiana stirred intense controversy with remarks about rape and abortion. Akin suggested that pregnancy rarely results from "legitimate rape," while Mourdock said that even pregnancy from rape was a life that "God intended."
Missouri's Democratic Sen. Claire McCaskill, who had once been a prime GOP target as an ally of Obama, won after Akin's comments narrowed the race. In Indiana, Rep. Joe Donnelly switched the Senate seat into the Democratic column after winning a long-shot bid against Mourdock.
One key GOP-held seat was in Nevada, where Sen. Dean Heller, who was appointed to office after his Republican predecessor resigned amid a sex and lobbying scandal, beat back a challenge from Democratic Rep. Shelley Berkley, the congresswoman from Las Vegas.
The Republican strongholds of Nebraska and North Dakota had been considered easy flips to the GOP column with the retirement of Democratic senators.
In Nebraska, former Democratic Sen. Bob Kerrey, once the governor, was unable to make a comeback against Republican Deb Fischer, a Sarah Palin-backed state legislator. But Democrat Heidi Heitkamp, the former North Dakota attorney general, proved to be a robust campaigner and edged out Republican Rep. Rick Berg, although he could demand a recount.
A record number of women ran for the Senate. Hawaii's Democratic Rep. Mazie K. Hirono defeated former Gov. Linda Lingle, a Republican, to become the first Asian American woman in the Senate.
Perhaps one race brought the most uncertainty: In Maine, the independent former governor, Angus King, has declined to say which party he would caucus with, although he is expected to join Democrats. On Tuesday, he claimed the seat held by retiring Republican Sen. Olympia Snowe, one of the few moderates in Congress.