Mitt Romney has had a pronounced change of fortunes since the first presidential debate in Denver. After trailing President Obama by 4 or 5 points in the polls on Oct. 1 — a position that very few candidates have come back from — he now holds ties or small leads in many national polls and has cut the advantage Mr. Obama had in swing states to a razor-thin margin.

There is little sign, however, that Mr. Romney’s rebound has translated into races for the Senate. Although Republicans have made modest gains in a few Senate races, the polls have been poor for them on the whole. Some races have already gotten away from them, while others are on the verge of being lost.

The FiveThirtyEight forecast model now gives Republicans just about a 16 percent chance of winning control of the Senate. This is a precipitous drop from just two months ago. On Aug. 19, the forecast put their odds at close to 62 percent.

Emblematic of Republicans’ problems is Florida, a state where Mr. Romney has made considerable gains in the polls and where their Senate candidate, Representative Connie Mack, had drawn nearly even this summer with the Democratic incumbent, Bill Nelson. But Mr. Mack fell behind in the polls in September, and two of the four polls published since the presidential debate had Mr. Nelson with a double-digit lead. Another poll showed Mr. Mack down by 5 points, despite giving Mr. Romney a 7-point lead in the presidential race.

The Republican candidate in Ohio, State Treasurer Josh Mandel, has followed a similar trajectory to Mr. Mack, with most surveys showing the Democratic incumbent, Sherrod Brown, with a solid lead. In Michigan, the Republican candidate, former Representative Peter Hoekstra, fell behind in the race early and now lags by double digits.

Three other races remain highly competitive but are showing the Democratic candidate with a more consistent lead in the polls.

One case is Massachusetts, where the Democratic challenger, Elizabeth Warren, has led in four of five polls since the presidential debate. In Wisconsin, the Democrat, Representative Tammy Baldwin, has led in all post-debate surveys.

The polling has been more varied in Virginia, a battle between a Democratic former governor, Tim Kaine, and a Republican former senator, George Allen. But Mr. Kaine pulled slightly ahead in the polls in September and has continued to lead in a majority of post-debate polls.

There have been no polls of Missouri taken since the debate, but two conducted just before it had the incumbent, Claire McCaskill, with a 6-point lead over her Republican challenger, Representative Todd Akin.

Republican candidates are also struggling to hold their ground in other races that once appeared to be likely wins for them. The most recent poll of North Dakota showed an exact tie between the Democrat, Heidi Heitkamp, and the Republican, Representative Rick Berg.

There have been few high-quality polls of the Senate race in Arizona, but the available ones suggest an extremely close race, even though the Republican, Representative Jeff Flake, was once favored to beat the Democrat, Richard Carmona.

The FiveThirtyEight Senate forecasts still list the Republicans as favorites in Arizona and North Dakota given the states’ history of voting Republican. But it has a third red-leaning state, Indiana, as a dead heat between the Republican, State Treasurer Richard Mourdock, and the Democrat, Representative Joe Donnelly.

Republicans have gotten better news in a few Senate races. In Nevada, Senator Dean Heller continues to hold the lead over the Democrat, Representative Shelley Berkley. In Connecticut, Linda McMahon, the Republican candidate, has remained competitive against the Democrat, Representative Christopher S. Murphy, although Mr. Murphy led in the only post-debate poll.

The best news for Republicans, however, may be Mr. Romney’s gains. If he wins the Electoral College, they would be able to control the Senate with a net gain of three seats since the new vice president, Paul D. Ryan, would then cast the tiebreaking vote.

Republicans’ best route to pick up three seats would be in Nebraska, where they are nearly assured of a win, and North Dakota and Montana, where the FiveThirtyEight model has the Democratic incumbent, Jon Tester, as a slight underdog to his Republican challenger, Representative Denny Rehberg.

But Republicans are likely to lose a seat in Maine, where the independent candidate, former Gov. Angus King, remains well ahead in the polls and is likely to caucus with Democrats if he wins. And in Massachusetts, the Republican senator, Scott P. Brown, is now the underdog to Ms. Warren.

Even if Republicans avoided taking any other losses — and even if Mr. Romney won the presidency — they would then need to find two more Democratic seats. Connecticut, Virginia and Wisconsin remain the most likely possibilities, but the Republicans are modest underdogs in each race.

And if Democrats win the tossup races, they could actually gain Senate seats. Only Nebraska looks like a sure loss for them, while Arizona, Indiana, Maine, Massachusetts and Nevada are all potential pickups.

That could bolster Democratic prospects for 2014, when Democrats will again have more incumbents up for re-election and will face a challenging climate in states like Alaska and Louisiana.