The battle for control of Congress rolled into a frenetic final weekend as Democrats fought to preserve the Senate as their power center on Capitol Hill, trying to hold off a Republican surge that could reshape the political order in Washington.
With Republicans in a strong position to capture the House, President Obama on Saturday opened a four-state weekend swing here to rally support for Senate candidates in Connecticut, Illinois, Ohio and Pennsylvania, hoping to build a critical firewall to protect the party’s Senate majority from Republican gains across the country.
Republicans intensified their efforts to capitalize on a favorable political environment, with Sarah Palin making a last-minute trip to West Virginia to ask voters to elect a Republican for the Democratic seat Senator Robert C. Byrd held for 51 years.
The outcome of five contests considered tossups will help determine if Democrats retain control of the Senate, according to the latest analysis of races by The New York Times, with Republicans trying to capture Democratic-held seats in Colorado, Illinois, Nevada, Pennsylvania and Washington. Should they sweep those, they would still need to triumph in a state like California or West Virginia, where Democratic chances seemed to be improving.
The analysis suggests that Republicans are on the cusp of significantly expanding their presence in the Senate, but will need almost everything to go their way on Tuesday to gain the 10 seats needed to win control.
A sour political environment has left almost no Democratic senator on the ballot immune to forceful challenges by Republicans. Polling shows that Republicans have a firm grip on open Democratic seats in North Dakota and Indiana. And Senators Russ Feingold of Wisconsin and Blanche Lincoln of Arkansas have fallen behind in their races, strategists in both parties said, while Harry Reid of Nevada and Patty Murray of Washington are trying to survive tough fights.
No contest in the country held a higher symbolic priority for Democrats or Republicans than that of Mr. Reid, the majority leader. He is deadlocked in a deeply personal multimillion-dollar battle with Sharron Angle, who has become one of the most viable candidates to ascend from the Tea Party movement.
The president made an urgent appeal to save the Democratic majority, including his own former Senate seat in Illinois. He returned to Chicago on Saturday evening for a rally in the neighborhood where his political career began, telling voters along the way that this election should not be about him.
“It is absolutely critical that you go out and vote,” Mr. Obama said here in Philadelphia. “This election is not just going to set the stage for the next two years. It’s going to set the stage for the next 10, the next 20.”
A new sense of uncertainty coursed through several other races, including the one in Alaska. Democrats began running TV advertising on Saturday to try to take advantage of a chaotic situation where Senator Lisa Murkowski, a Republican, is running an aggressive write-in campaign to try to overcome her loss to Joe Miller in the primary.
The Democratic candidate, Scott T. McAdams, seen as a long shot a few weeks ago, could have a chance if Ms. Murkowski and Mr. Miller split the votes of Republicans and independents.
Ms. Palin, the former governor of Alaska, who has been campaigning for Mr. Miller and other candidates, arrived in Charleston, W.Va., on Saturday afternoon to rally support for the Republican Senate candidate, John Raese, whose early momentum appears to have waned in his race against Gov. Joe Manchin III, a Democrat.
While Republicans continued to expand the battleground in the House, growing increasingly confident in their quest to win control of at least one chamber of Congress, Senate Republicans sought to contain expectations even though they will end the year in a much stronger position than they could have imagined two years after Mr. Obama entered the White House on a wave of popularity and optimism.
“Our hand will be strengthened, even if we’re not in the majority in January,” said Senator John Cornyn of Texas, chairman of the National Republican Senatorial Committee. “It’s remarkable that we’ve hung together as much as we have.”
Even as Election Day draws near and huge get-out-the-vote programs are waged on both sides, millions of voters have already cast their ballots, a phenomenon that has substantially changed how American elections are conducted. Democrats hoped the early-voting option in many states would help close the enthusiasm gap with Republicans, because party organizers could all but drag their voters to the polls over a span of weeks.
“It’s like a dogfight in each and every one of those states,” said Senator Robert Menendez of New Jersey, chairman of the Democratic Senatorial Campaign Committee. “We are poised to surprise people on election night, but we are in a dogfight.”
In Philadelphia, Mr. Obama’s arrival was intended to draw attention to the Senate contest pitting Representative Joe Sestak, a Democrat, against Pat Toomey, a Republican and former congressman. In the last two weeks, Mr. Sestak appears to have cut into Mr. Toomey’s lead. Even though Democrats hold a statewide voter registration edge of 1.2 million people, party strategists worry that a wave of protest against the party in power will propel Republicans.
At a crisp morning rally, Mr. Sestak said it was time for voters to move beyond the anger at Washington and focus on the substantive differences between the two parties. He urged Democrats to think about the consequences of Republicans winning control of Congress, particularly in upholding fair trade policies and trying to repeal the health care law.
“I think it will be very tight, but I do believe there is just enough time,” Mr. Sestak said. “We’re going to prevail because of the common sense of Pennsylvania.”
Mr. Toomey worked the critical Philadelphia suburbs on Saturday, appealing to independent and Republican voters, even as his campaign released a list of prominent Democrats who are supporting his candidacy, a sign that disaffected Democratic voters are up for grabs.
In Colorado, Democrats were hoping Senator Michael Bennet, appointed to the Senate last year, could win his close fight with Ken Buck, a Republican, and give Democrats some insurance in their push to retain the majority and keep some of the territory the party won in the 2008 election in the Mountain West.
“Colorado is an independent state, and I think we are seeing that in this election, as close as it is,” Mr. Bennet said in an interview as he made a final tour of the state. “We are going to need Democratic votes, unaffiliated votes and Republican votes, and I think we can do it. We have run a very strong campaign at a tough time.”
Republicans appear to be in no danger of losing their party’s open Senate seats in Kentucky, Missouri, New Hampshire and Ohio, though just last year it seemed Republicans would have difficulty keeping all of them.
The difficult climate for Democrats and the strong Republican momentum around the nation has made simply holding on to the Senate seem like a victory even though the party stands to lose a significant chunk of its majority, some senior lawmakers and perhaps the majority leader.
When control of the House flipped in the 1994 and 2006 elections, power in the Senate changed hands simultaneously as the Republican surge in the first case and the Democratic tide in the second carried across the Rotunda. With Republicans in a commanding position to take the House but a Senate majority still in doubt, that pattern could well be broken this time.
Republicans and Democrats cite several factors for the potential disparity between the House and Senate races. One reason is that Republicans began the election cycle needing 11 seats to take the majority, a number reduced to 10 after Scott Brown, a Republican, won a special election in Massachusetts in January.
Republicans now have 41 senators and would need to win 10 seats to grab control, as Democrats would hold the edge in a 50-50 Senate because of the tie-breaking ability of Vice President Joseph R. Biden Jr. Even in a highly favorable year for Republicans, 10 seats is a tall order.
“We were at a low point after two losses in a row,” said Senator Mitch McConnell of Kentucky, the Republican leader. “We had more ground to make up.”
Jeff Zeleny reported from Philadelphia, and Carl Hulse from Washington.
This article, , first appeared in The New York Times.