WASHINGTON, Nov. 4 — The battle for Congress rolled into a climactic final weekend with Republican Party leaders saying the best outcome they could foresee was losing 12 seats in the House. But they were increasingly steeling themselves for the loss of at least 15 seats and therefore control of the House for the first time in 12 years.

Democrats and Republicans said the battle over the Senate had grown fluid going into the final hours before the elections on Tuesday. Democrats said they thought they were almost certain to gain four or five seats and still had a shot at the six they need to take control.

Republicans were pouring money into Senate races in Michigan and Maryland this weekend to take advantage of what they described as last-minute opportunities, however slight, in states currently held by Democrats. And a new poll Saturday showed that Senator Conrad Burns, the Montana Republican, was tied with his challenger after a visit there by President Bush.

Party strategists on both sides, speaking in interviews after they had finished conducting their last polls and making their final purchases of television time, said they were running advertisements in more than 50 Congressional districts this weekend, far more than anyone thought would be in play at this stage.

Nearly all of those seats are held by Republicans, underscoring the degree to which President Bush and his party have been forced onto the defensive two years after he claimed that his re-election had given him the political capital to carry out an ambitious domestic and foreign agenda.

"It's the worst political environment for Republican candidates since Watergate," said Glen Bolger, a Republican pollster working in many of the top races this year.

As the final weekend began, the parties made final tactical moves as candidates sparred over the war, the economy, corruption and competence and as elaborate get-out-the-vote campaigns were rolled out. At stake was not just control of the House and the Senate, but also potentially the course of the Bush presidency in its last two years and the debate over how to proceed in Iraq.

Democrats bought advertising time in yet another House race that had long been considered safe for Republicans, that of Representative Marilyn Musgrave of Colorado. Mr. Bush appeared at a rally in Ms. Musgrave's district on Saturday morning, part of a late flurry of campaigning by the president aimed at shoring up struggling Republicans in some of the reddest states in the country, including Nebraska and Kansas.

In another bit of news that sent a chill through many Republicans, a University of New Hampshire poll showed Representative Charles Bass, a popular moderate Republican from New Hampshire who had not been seen as vulnerable this year, trailing his opponent after Democrats spent $1 million in his district.

In upstate New York, Representative John E. Sweeney, a Republican who had seemed to be weathering a tough challenge, was described by party strategists as in new danger after his local newspaper, the Glens Falls Post Star, withdrew its endorsement of him, citing reports of a domestic violence episode involving Mr. Sweeney and his wife.

Joe Gaylord, who was the political lieutenant to Newt Gingrich when he led the Republican takeover of the House in 1994, said that based on polling he had seen in recent weeks, he expected his party to lose 25 seats to 30 seats on Tuesday. That general assessment was repeatedly echoed in interviews with Republicans close to the White House.

"It's very grim," Mr. Gaylord said. "Things are dreadful out there."

Representative Thomas M. Davis III, a Virginia Republican and veteran party strategist, said: "There's no question we're going to take a hit. The only question is how hard it would be."

Still, some Republicans and Democrats said that Republicans could be bolstered by structural advantages that could, at the very least, minimize the party's losses. Aides to both parties said that at least 20 races were close enough that struggling Republican incumbents could be pulled to victory by the party's sophisticated get-out-the-vote operation.

Karl Rove, Mr. Bush's chief political adviser, has assured nervous associates that the Republican turnout operation would help save the party from electoral disaster.

Representative Thomas M. Reynolds, the New York Republican heading his party's effort to hold onto the House, said: "Turnout will be key to us in these three dozen races that are close across the country." In a sign of how unexpectedly challenging the climate is for Republicans, Mr. Reynolds has had to devote much of his time to his own tough re-election battle.

For all the deep unhappiness that polls show with Congress, Mr. Bush, his party and the Iraq war, only about 10 percent of House races could be considered even remotely competitive. That figure stands as a reminder of the enduring power of incumbency, and of how a dominant party can protect itself by drawing Congressional districts that serve as bulwarks during stormy seasons. There are 34 incumbent House members and one senator running for re-election unopposed.

"If the Democrats end up with 53 percent of the national vote and still don't get a majority in the House, which is conceivable, it's a clear sign that this Republican structural advantage has really kicked in," said Gary C. Jacobson, a political scientist at the University of California, San Diego.

Ken Mehlman, the Republican national chairman, said in an interview that the "race for the House remains very close, and I believe we will keep our majority.

"And I think the Senate, in the last week," he said, "has been very good for Republicans and very difficult for Democrats."

His Democratic counterpart, Howard Dean, offered a different reading of the electoral environment. "The president is spending the last Sunday of the campaign in Nebraska!" he exclaimed, alluding to that state's largely Republican status. "Who would have ever thought that?"

Appropriately enough for a campaign that has been marked by searing negative advertisements, the campaign-closing round of advertising — which in more typical years has consisted largely of gauzy 30-second spots in which the candidate makes an earnest plea for support — was this weekend led by another round of dark charges on topics including coddling terrorists and raising taxes.

"Drunk Driving. Arrests. Federal investigations," an announcer says in a new advertisement aimed at Mr. Sweeney by his opponent, Kirsten Gillibrand.

Across the country, but particularly in Missouri, Ohio, Pennsylvania and Montana, the massive voter turnout operations began rolling into their final 72-hour plans, with one more round of telephone calls and personal visits to voters who had long ago been identified. Volunteers bused in from around the country — the roll call of states ranged from to New Jersey to Mississippi— assembled at the Ohio Democratic Party in downtown Columbus Saturday morning, to gather literature, lists and their marching orders for the day.

The tautness of this midterm election was in evidence across the country. Representative Deborah Pryce, an Ohio Republican, could barely hide her frustration Saturday as she feverishly went table to table in a restaurant looking for support.

"It's been a tough, awful race," Ms. Pryce said to a table of voters.

"Well, we are concerned for you," responded Donna Beachy, 76, of Plain City. "The Democrats are really out working hard. We're getting three calls a night, all with recordings, and most of them are from Democrats."

In the Senate, Republicans girded themselves for what strategists from both parties described as the almost certain defeat of Senators Mike DeWine of Ohio and Rick Santorum of Pennsylvania. They said that Senator Lincoln Chafee of Rhode Island was also probably headed for a loss. Democratic hopes of winning a seat in Tennessee seem to have faded in recent days, while their chances of unseating Senator George Allen of Virginia appear to be on the rise, leaders in both parties said.

But Mr. Burns, a Republican who has been struggling all year in the face of a tough challenge by Jon Tester, was said by aides in both parties to have grown stronger in recent days. A poll conducted by Mason-Dixon for Lee Newspapers and published on Saturday found that Mr. Burns and Mr. Tester each had the support of 47 percent of the voters.

Republicans said they were still hopeful of unseating Senator Robert Menendez, Democrat of New Jersey, after pouring $5 million into his state over the past 10 days. But some analysts said the onslaught might prove to have been too late to make a difference; a WNBC/Marist Poll released Saturday evening showed Mr. Menendez with an eight-point lead over his Republican opponent, Tom Kean Jr.

In what some senior Republican strategists said was something between a long-shot and a Hail Mary pass, Republicans were spending money in Michigan to defeat Debbie Stabenow, the Democratic incumbent, as well as in Maryland, hoping that black voters in the state would desert the Democratic Party and vote for Michael Steele, a black Republican running for an open seat.

Senator Charles E. Schumer, the New York Democrat leading the party's effort to win the Senate, said : "I would say it's going to be very close one way or the other. The odds of it being four, five or six seats are higher than it being three or seven."

In the House, Democrats seem all but assured of picking up open Republican seats in Arizona, Iowa and Colorado, along with the Ohio seat of Bob Ney, who pleaded guilty to corruption charges and stepped down on Friday.

Party leaders said the Republicans who seemed headed for defeat on Tuesday included Representatives Curt Weldon of Pennsylvania, and Chris Chocola and John Hostettler of Indiana. Other likely Democratic pickups include the Florida seat being vacated by Katherine Harris, who is running for Senate, and the upstate New York seat being vacated by Sherwood L. Boehlert.

Officials in both parties said more than 20 House contests remained very tight, which is why Democrats could end up capturing at least 30 seats or falling short of the 15-seat gain they need to take control, depending on turnout and last-minute shifts. A number of respected independent analysts, including Stuart Rothenberg and Charles Cook, have predicted that Democrats could gain 35 seats or more.

It is possible that no Democratic incumbent will be knocked out of office, though Republicans have made a concerted effort to unseat two Democrats in Georgia.

Faced with diminishing opportunities, Republicans have continued through this weekend to spend money to hold on to two other seats vacated by Republicans who left under clouds: Mark Foley of Florida and Tom DeLay of Texas. The fact that the party had to pour resources into those two races, along with others in which incumbents had been hurt by their own scandal or tainted by the fallout from the Foley Congressional page scandal, was said by Republican strategists to be one of the biggest problems they faced.

"The scandal seats have clearly hurt us and put us at a disadvantage," said Carl Forti, a senior strategist with the National Republican Congressional Committee.

Carl Hulse and Jeff Zeleny contributed reporting.

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