Republicans' once-bright prospects for taking over the U.S. Senate in November are slipping away as President Obama expands his lead over Republican Mitt Romney, boosting other Democrats on the ballot.

Obama and Romney remained virtually tied in national polls, but surveys show Obama leading in a handful of crucial swing states, weakening Republican hopes in close Senate races in Massachusetts, Virginia, Florida and Wisconsin, among others.

"Republicans still have a chance, but it's not what it was six months ago or a year ago," Stu Rothenberg, editor of the Rothenberg Political Report, told The Washington Examiner. "And if you had to weigh the two parties chances, you would have to say the Democrats have a better chance of holding the Senate."

Democrats have just a four-seat majority in the Senate, and 23 Democrats face re-election this November compared to only 10 Republicans. Still, Democrats have strengthened their position in a number of previously vulnerable seats and now have a shot at capturing four of the 10 Republican seats.

In Virginia, for example, recent polls shows Obama pulling ahead of Romney by 5 percentage points, according to RealClear Politics. While Obama won this historically red state in 2008, many political strategists predicted he would have a much harder time winning it again this year. Yet, Virginia voters so far appear to favor him over Romney and that's helping Democratic Senate candidate Tim Kaine pull ahead of his Republican opponent, George Allen.

A Fox News poll released last week showed Obama pulling ahead of Romney in Virginia and Kaine taking a 4-percentage-point lead over Allen, the first time either Senate candidate has pulled ahead in a race that's been deadlocked for over a year.

"It's almost inconceivable that Obama could win even by a point or two without Kaine also winning," University of Virginia political science professor Larry Sabato told The Examiner. "Practically speaking, there just aren't a whole lot of Obama-Allen voters."

Sabato and other political experts expect a similar "coattail effect" in a number of other states, including Massachusetts, that are crucial to the fight for control of the Senate.

In Massachusetts, Democrat Elizabeth Warren surged ahead of incumbent Republican Sen. Scott Brown, polls show, and Bay State strategists credit her rise to the bounce Obama got coming out of his party's convention -- a convention in which Warren raised her own profile by delivering a keynote speech. Warren and Brown were tied before the convention.

"The common denominator in all the polls is that the convention helped her immensely," Democratic strategist Mary Anne Marsh told The Examiner.

Obama is also helping boost Democratic Senate candidates in Wisconsin and Florida. And Democratic incumbent Sen. Claire McCaskill of Missouri is now much safer since her Republican opponent, Rep. Todd Akin, alienated many Republicans by claiming "legitimate" rape victims can't get pregnant.

But Romney does appear to be reversing Democratic gains in one state, Indiana, where the Senate race remains a tossup. Obama won Indiana in 2008, but Romney now leads there. And that's helping Republican Senate candidate Richard Mourdock hold Democrat Joe Donnelly in check, Mourdock's campaign aides said.

"Polling we've seen here shows Romney is doing much better here than he is around the country relative to the trend lines," Deputy Campaign Manager Brose McVey said. "So that's helpful."