WASHINGTON » The fight for the Senate has shifted significantly over the past weeks, with fierce races breaking out in states where they were not expected and other contests dimming that were once ablaze with competition.

With less than two months until Election Day, the Senate landscape is both broader and more fluid than it has been in years, with control of the upper chamber now anyone’s guess. Both parties have seen new opportunities and new challenges, but the net result is that Democrats appearto be in less danger of losing the Senate, while Republicans have a more difficult path to gaining the majority.

Connecticut may be the biggest surprise. Two years after a decisive loss in her first Senate campaign, the Republican candidate, Linda E. McMahon, a former professional wrestling executive, is surging in polls. Wisconsin is also now tilting Republican. Democrats face blistering advertisements financed by super PACs in states they once thought were secured, and the tight presidential contest in swing states like Ohio, Florida and Nevada is keeping Senate races there closer than anticipated for both parties.

Democrats are now strongly competitive in races for the Republican-held seats in Indiana and North Dakota, where the Republican candidates — who were expected to walk away with those races — have exhibited weakness.

"The map is bigger now than I’ve ever seen it at this point in an election," said J.B. Poersch, a longtime Democratic Senate campaign strategist who is now with Majority PAC.

The has updated its assessment of the Senate races. Indiana, where the Republican nominee, state Treasurer Richard E. Mourdock, is competing against Rep. Joe Donnelly, has turned from a safe Republican seat to one that leans Republican. Connecticut has moved from a safe Democratic seat to one that leans Democratic, and Wisconsin, which was a tossup, now leans Republican.

Republicans need a net gain of four seats to win control of the Senate, but what was once a good bet that they would do so is now a coin toss.

"A year ago, I thought the Republicans were certainly more likely than not to net four seats and win control," said Stuart Rothenberg, editor of the nonpartisan Rothenberg Political Report. "It increasingly looks like they have to run the table here."

Some of the campaign missteps and unexpected turns are well documented. In Maine, Sen. Olympia J. Snowe’s surprise retirement this year almost certainly removed a seat from the Republicans’ column.

Rep. Todd Akin’s comments about "legitimate rape" may well have robbed Republicans of a Missouri Senate seat they had counted as theirs, and have improved the chances that Sen. Claire McCaskill, the Democratic incumbent, will win re-election.

Broader political forces have conspired to seemingly move some seats out of the Republicans’ reach.

With former Rep. Heather Wilson, the Republicans in New Mexico got the moderate veteran candidate they wanted. But Wilson appears to be getting no help from Mitt Romney in the state, where the Hispanic vote can be decisive. This month, the National Republican Senatorial Committee quietly canceled advertising there to shift those resources elsewhere.

Taking advantage of that move, the Democratic Senatorial Campaign Committee has cautiously followed suit, according to an official familiar with the group’s moves. On Monday, the committee canceled a week’s worth of advertising in New Mexico. It still has about $1.8 million in reserved advertising time this fall in case Wilson mounts a comeback.

In other races, the candidates themselves have become a problem. In Indiana, Mourdock, a Tea Party favorite, defeated Sen. Richard G. Lugar in the Republican primary for a seat that he had held since the mid-1970s. But Mourdock has struggled, and Donnelly has gained traction, drawing support from some of Lugar’s backers. Donnelly will have to rely on a brew of support from conservative Democrats, independents and disaffected Republicans who would never think of voting for President Barack Obama but who might be turned off by Mourdock’s anti-compromising attitude, which has not mellowed much during the general election.

"Going back 25 and 30 years, there is a history of ticket-splitting in the presidential year with Hoosiers," said Dan Parker, chairman of the Indiana Democratic Party.

Democrats have their own issues. In Wisconsin, they were hoping a tough Republican primary would beget a Tea Party-aligned conservative who would have trouble winning statewide, as it did in Indiana.

Instead, Tommy Thompson, the former Wisconsin governor and health and human services secretary under President George W. Bush, prevailed. His Democratic opponent is Rep. Tammy Baldwin of Madison, the state’s liberal bastion, and many predict she will be a tough sell in more conservative parts of the state. Polls have given Thompson an early lead, and Romney’s decision to choose Rep. Paul D. Ryan of Janesville, Wis., as his running mate has complicated the Democrats’ campaign, though Obama’s popularity in the state should help Baldwin.

Then there are the candidates whose early promise has faltered, even in states where party registration favors them.

In Massachusetts, the Democratic nominee, Elizabeth Warren, a Harvard University professor and creator of the Consumer Financial Protection Bureau, has raised record amounts of money. Yet in a state that is likely to vote overwhelmingly for Obama, she cannot pull away from Sen. Scott P. Brown, a Republican, whose personal appeal remains strong among voters.

Similarly, in North Dakota, where Romney will win in a walk, likability is a major factor in Rep. Rick Berg’s struggles with this Democratic opponent, Heidi Heitkamp, whose name has not appeared on a ballot since losing the governor’s race 12 years ago.

"Republicans would have said North Dakota is done by Labor Day," said Jennifer Duffy, who analyzes Senate races for the nonpartisan Cook Political Report. "Democrats would have said the same thing in Massachusetts."

Nebraska Democrats pressed Bob Kerrey, a Vietnam War hero, to run for his old Senate seat. He has made little headway.

In Connecticut, McMahon, who lost badly in her 2010 Senate race despite spending millions of dollars of her own fortune, is gaining more traction this time as she seeks the seat being vacated by Sen. Joseph I. Lieberman. In late August, a Quinnipiac University poll of likely voters put her slightly in the lead over her Democratic opponent, Rep. Christopher S. Murphy.

McMahon emphasized her business expertise two years ago. She has sought to soften her image this time around, stressing her role as a mother and grandmother. At the same time, Murphy is weathering a spate of bad headlines for failing to pay his mortgage and rent in the past.

"It’s a big thing for women in this state to see themselves in their candidate," said Todd Abrajano, a spokesman for McMahon’s campaign.

If nothing else, McMahon is forcing Democrats to spend precious resources in an state that is part of the expensive New York media market. Rep. John B. Larson, D-Conn., said he had secured assurances from Sen. Charles E. Schumer of New York, a senior member of the Democratic leadership, that Murphy would have the money he needed.

Money is the great unequalizer. In Ohio, Sen. Sherrod Brown seemed far ahead a few months ago, but at least $15 million worth of advertisements paid for by outside groups on behalf of his Republican opponent, Josh Mandel, is tightening up the race. The story is similar for Sen. Bill Nelson of Florida, a Democrat who maintains a lead that money may cut into.