Only 15 months after being left for dead, the Republican Party suddenly finds itself not just with a pulse, but within reach of reclaiming the U.S. Senate.

After Indiana Sen. Evan Bayh's surprise announcement Monday that he will not seek re-election to a third term in November, the Cook Political Report declared the Democrat's seat is now leaning Republican, and the Rothenberg Political Report moved the seat from narrow advantage for Democrats to a "tossup," adding that Bayh's decision "gives Republicans another excellent takeover opportunity."

The Republicans, who hold 41 seats, have a shot to grab as many as 10 that are considered vulnerable in November. If the GOP can run the table, it will strip the Democrats of their majority.

"I'm feeing pretty bad," Democratic pollster and Fox News contributor Doug Schoen said about Bayh's retirement decision and its political implications.

"I think there are eight if not nine seats in play," Schoen said. "Evan Bayh's retirement is a message to centrist voters that you're not welcome in the Democratic Party. And if Democrats don't get centrists and independents, they can't hold the Senate and they can't win elections."

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Schoen said centrists in the Democratic Party are an "endangered species that could well be all but eliminated by November."

But the campaign arm of the Democratic Party isn't ready to wave a white flag.

"There's no question Democrats face a strong headwind in November, but given the 12 bloodletting Republican primaries, their Washington-insider candidates like Roy Blunt and Rob Portman, their battered brand, and their anemic poll numbers, Republicans may  be a bit premature in their orders of champagne by the truckload," Eric Schultz, a spokesman for the Democratic Senatorial Campaign Committee, told Fox News.

"This election will likely turn into a choice between Republicans who are standing with Wall Street fat cats, bankers and insurance companies -- or Democrats who are working hard to clean up the mess we inherited by putting the people's interests ahead of the special interests," he said.

Republicans, meanwhile, were trying to remain level-headed.

"It's clear that moving into 2010, the wind is at Republicans' backs. ...That being said, the election is still months away and our side is taking nothing for granted," Brian Walsh, spokesman for the National Republican Senatorial Committee said. 

"Republican Senate candidates will continue to listen to voters and offer positive, common-sense alternatives to the big spending, big government bills being pushed by Democrat candidates," he said. 

In an effort to halt GOP momentum, President Obama is heading west to campaign for Senate Majority Leader Harry Reid of Nevada and Michael Bennet of Colorado, both of whom are facing challenging contests.

On Thursday, Obama will stump for Bennet, who was appointed to the seat after the president tapped Ken Salazar as his secretary of the interior.

Bennet trails one opponent by 14 percentage points, according to a Rasmussen poll.

On Friday, Obama will stump for Reid, whose approval ratings have plummeted to record lows. Three Republican challengers are beating Reid in a variety of polls.

The prospects for the Republicans against other Democratic incumbents look just as good in some other states.

Of the 10 Democratic seats in play, the Cook report regards five as tossups -- Arkansas, Colorado, Illinois, Pennsylvania and Nevada. The report says two -- California and Connecticut -- are leaning Democrat, and it picks two others -- Delaware and North Dakota -- as slam dunks for Republicans.

By contrast, the report considers only four Republican seats as tossups: Kentucky, Missouri, New Hampshire and Ohio.

In all, the Rothenberg report lists eight Democratic seats as vulnerable. Arkansas, Nevada, North Dakota and Delaware are leaning Republican; Illinois, Indiana, Colorado and Pennsylvania are considered tossups.

Fox News' Trish Turner contributed to this report.