As Mitt Romney sank in the polls in September, so did the fortunes of many Republican Senate candidates, seemingly putting control of the upper chamber out of the party’s reach.

But Mr. Romney’s strong debate performance last week gave him a boost, and now Republicans are eagerly awaiting the next round of Senate polls, banking that their presidential nominee’s rise will lift the rest of the party’s ticket.

“It was always going to be a close election for the [Democratic Senate] majority,” Republican strategist Ron Bonjean said. “However, the better Romney does, the better off the ground is for Republicans in these states.”

Political scientists have long debated presidential candidates’ coattails — their ability to help boost party members’ fortunes down the ticket. But there is little question that as Mr. Romney’s poll numbers dropped in September, so did support for Republican Senate candidates in swing states such as Virginia and Nevada.

Now, as Mr. Romney has closed the gap in national and battleground-state polls, some of those Republican Senate candidates are making a bit of a comeback of their own.

In Virginia, Democrat Tim Kaine’s lead has slipped, according to the Real Clear Politics average of polls. The same has happened to Sen. Sherrod Brown, the Democratic incumbent in Ohio. In Nevada, Sen. Dean Heller, a Republican, has increased his lead over Rep. Shelley Berkley, a Democrat who is trying to unseat Mr. Heller.

Yet in Michigan, Sen. Debbie Stabenow, a Democrat, has extended her lead into double digits, and in New Mexico Rep. Martin Heinrich, also a Democrat, has built on his advantage.

Political scientist Larry J. Sabato, a professor at the University of Virginia, said the Romney bounce is not related to changing Senate fortunes — at least not yet.

“Right now these [Senate] contests are running on different tracks,” he said. “Sometimes you have [presidential and Senate races] on parallel tracks. I don’t think these are right now. But they will be to a greater extent on Election Day.”

Mr. Sabato said Republican Senate candidates whose poll numbers dropped in September have no one to blame but themselves.

“They’re responsible for their own campaigns. I don’t see how they can link that to Romney in September,” he said.

Likewise, Jim Manley, a Democratic strategist and longtime Senate aide who now works for public affairs firm Quinn Gillespie & Associates, said Senate candidates controlled their own destinies in September — to the benefit of his party.

“In many of the races, [Democrats] have better candidates than what we had expected,” he said.

But former Rep. Thomas M. Davis III, who represented Northern Virginia and was chairman of the National Republican Congressional Committee for two election cycles, said Mr. Romney’s September slump absolutely hurt his party’s down-ticket candidates.

“If Romney wins the presidency, Republicans are likely to take the Senate,” he told The Washington Times.

Last month, speaking to reporters in the midst of Mr. Romney’s slump, he said whether George Allen, a Republican, recaptures his Senate seat or loses to Mr. Kaine depends on the outcome at the top of the ticket.

“Romney, pure and simple,” Mr. Davis said. “It’s true everywhere.”

Mr. Davis, who is now chairman of the Republican Main Street Partnership, an umbrella group for Republican centrists, noted that Mr. Allen last faced voters in 2006, and Mr. Kaine last faced them in 2005, when he won his term as governor.

Since then, so many people have moved into metropolitan areas such as Northern Virginia — and whichever party captures their presidential votes likely will win their Senate votes, too.

Republican strategist David Winston said the catalyst of Mr. Romney’s surge was his ability to make the nation’s improving but soft economy a focal point of the debate. If Mr. Romney continues to do that until Election Day, he said, Republican candidates for Senate will share in his success.

“If [the race] is about other things, then all of a sudden this becomes personality-based and not necessarily focused on jobs and the economy. That probably is an environment that is helpful to Democrats in the sense that then they can sort of localize all these individual [Senate] races,” he said.

Mr. Sabato said Mr. Romney can grow coattails if he notches two more wins in the debates.

“He’ll have an Election Day coattail [if] he can stretch this bounce out. And the only way to stretch it out is to win the other two debates,” the professor said.

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