Democrats maintained control of the U.S. Senate on Tuesday by snatching Republican Senate seats in Indiana and Massachusetts, and averting what was once considered a likely defeat in Missouri.

In Indiana, Rep. Joe Donnelly did what had seemed impossible by taking a Senate seat for the Democrats in a heavily Republican state, just weeks after his opponent - state Treasurer Richard Mourdock - said he believed that pregnancies that result from rape reflect the will of God.

Sen. Claire McCaskill of Missouri, a Democrat once considered the most endangered incumbent in the Senate, beat Rep. Todd Akin, who seemingly sank his campaign when he said women who are victims of “legitimate rape” would not get pregnant.

In Massachusetts, Elizabeth Warren, a Harvard professor, swept Republican Sen. Scott Brown from power. Democrats had longed to regain the seat that had been lost in a special election after Sen. Edward Kennedy’s death in a race that was dominatedby the national debate over the health-care overhaul.

Those Democratic triumphs followed quick wins in Ohio, Connecticut, Florida and Pennsylvania, all states where Republicans had harbored ambitions of victory that would propel them to a Senate majority for the first time since 2006.

Republicans lost in another state when former Gov. Angus King Jr. of Maine, an Independent, won his race to succeed Sen. Olympia Snowe, a GOP moderate who is retiring. King has yet to say which party he will caucus with next year, but he had warned Republicans and Democrats that his treatment during the campaign would bear on that decision. National Republicans and their super PAC allies responded by pummeling him with negative advertisements that did little to shake his lead.

“We said we’d defend all of our seats and would put half of their seats in play,” said Sen. Patty Murray of Washington, the chairman of the Democratic Senatorial Campaign Committee, who took that job last year when others had refused it.

“No one believed me,” she said, “but we did just that.”

After a series of hardfought and sometimes nasty battles in Senate races that spanned the country, little changed in the chamber: Democrats were likely to expand on their current 53-to-47-seat edge.

But neither party appeared to be in a position to gain the seats necessary to win a filibuster-proof 60-seat majority, meaning the possibility of a continuation of the gridlock that has been a hallmark of the modern Senate.

Still, a bare majority for Democrats offers them the chance to control the chamber’s agenda and committee structure. With that edge comes new leverage in negotiations over the nation’s most difficult problems, including fiscal issues that must be addressed even before the next Senate takes office.

With the GOP retaining control of the U.S. House of Representatives, Democrats needed to hold the Senate as a legislative ally to a re-elected President Barack Obama.

Two years after Tea Partybacked candidates in Colorado, Delaware and Nevada fumbled away Republican chances at Senate control, a new crop of conservatives appeared to do the same thing.

“It’s very important for the Republican Party to understand if we’re going to be a majority party beyond the House, we need to select candidates who can appeal to the electorate in all the states,” said Michael Castle, a moderate Republican and former congressman from Delaware who was supposed to cruise to a Senate seat in 2010. He lost the primary to Christine O’Donnell, who was backed by the Tea Party and then lost in the general election.

On Tuesday, Rep. Christopher Murphy fended off the deep-pocketed campaign of former wrestling executive Linda McMahon to win a Senate seat in Connecticut, and Sen. Bill Nelson of Florida defeated his Republican challenger, Rep. Connie Mack.

Sen. Sherrod Brown of Ohio held off Josh Mandel, the Republican state treasurer, weathering an onslaught of negative advertising from outside groups to keep a seat for Democrats in a presidential battleground that Republicans were counting on.

In New York, Sen. Kirsten Gillibrand, a Democrat, cruised to re-election. Sen. Robert Menendez, D-N.J., was also easily re-elected.

Montana and North Dakota - two conservative states that went for Republican Mitt Romney in the presidential election - were home to two of the nation’s most hotly contested Senate races.

In Montana, Sen. Jon Tester, a Democrat, faced Rep. Denny Rehberg. And in North Dakota, GOP Rep. Rick Berg faced a surprisingly tough battle from former state Attorney General Heidi Heitkamp to replace retiring Sen. Kent Conrad, a Democrat.

Republicans claimed one state currently held by Democrats - Nebraska. In that race, state Sen. Deb Fischer, a Republican, bested former Sen. Bob Kerrey in a race to replace retiring Sen. Ben Nelson, a Democrat.

In Wisconsin, Rep. Tammy Baldwin, a Democrat, beat former Republican governor Tommy Thompson, making the veteran the nation’s first openly gay senator.

In Pennsylvania, Democratic Sen. Robert Casey defeated coal executive Tom Smith, who spent more than $17 million of his own money on the race.

And in Nevada, Sen. Dean Heller was locked in a tight race against Rep. Shelley Berkley.

Democrats started the cycle with 23 seats to defend and the Republicans 10, an imbalance produced by the Democratic sweep of 2006. With only a three-seat majority for the Democrats, including two independents who caucused with them, holding on to control of the chamber seemed like an impossible task.

To defend some of the seats in heavily Republican states where Democrats were retiring, the party recruited talented candidates like Heitkamp, a former North Dakota secretary of state. They also pulled in strong candidates in Arizona, Indiana and Massachusetts, forcing the Republican Party to defend seats across a broader map in a year that was supposed to be all offense.

Also helping Democrats was the GOP primary defeat of Sen. Richard Lugar of Indiana, a Republican veteran who was expected to walk to re-election. Mourdock’s nomination turned the general election into a fight.

Information for this article was contributed by Jonathan Weisman of The New York Times and Rosalind S. Helderman of The Washington Post.

Front Section, Pages 10 on 11/07/2012