Former Rep. Pat Toomey officially threw his hat into the 2010 Pennsylvania Senate race Wednesday in a bid to unseat five term incumbent Sen. Arlen Specter.

"I am now a candidate for the U.S. Senate," the 47-year old Republican declared on a conservative radio show in his home town of Allentown. 

Specter has wasted no time in warning Republicans of the consequences of supporting Toomey, whom he claims is unelectable in a general election.

"Without Sen. Specter's seat in the Senate, which Mr. Toomey would certainly lose, there would no longer be 41 Republican senators to filibuster and stop the Democrats from passing card check, raising taxes, and implementing President Obama's massive spending plans," Christopher Nicholas, campaign manager for Specter, said in a statement Wednesday.

Specter and Toomey are no strangers to this fight. They waged a bitter war against each other in 2004, with Toomey losing by just over one percent of the vote. This time around it promises be just as rough, with the mud already flying.

Toomey, as former head of the anti-tax Club for Growth, has directed his fire at the moderate Republian for months, fanning the flames of populist anger at taxpayer-funded corporate bailouts of Wall Street fat cats and a multi-billion stimulus bill, a dangerous political recipe for the incumbent who supported these controversial measures. 

"I think our federal government has taken a very dangerous lurch to the left," Toomey said on the "Gunther Show." 

But Specter, ever the fighter, knew his votes were potential liabilities and launched a preemptive strike against Toomey's own resume, his experience as a Wall Street investment banker back in the late 1980's. The senator featured this line of attack earlier this month in a statewide television ad, a 2010 midterm first, with the primary still a year away and Toomey not even yet a declared opponent.

Central in Toomey's fight is Specter's support for the $787 billion stimulus bill earlier this year, a vote Specter defended this week alongside first responders at a police station in suburban Philadelphia. Indeed, Specter not only voted for the bill, he was central to the negotiated compromise that was approved by Congress with the support of only three Republicans in the end. 

"I know this may cost me politically, but I feel it is the right thing to do for the country in this economic crisis," Specter told FOX News at the time.

But the senator could survive the fight, according to one political expert, who says not to underestimate the business community's decades-old support for Specter.

"Sen. Specter has problems with three sets of Republicans that matter: social conservatives, supply siders (ideological conservatives), and some Republican party activists who think Specter's 60/65 percent lifetime record of voting with his party is too low and too disloyal," said Terry Madonna, director of the Center for Politics and Public Affairs at Franklin and Marshall College. 

"But he has always done well with the more practical business types--the Chamber of Commerce Republicans-- especially in the southeastern part of the state," Madonna said.

And that, Madonna said, is how Specter could overcome his perceived stimulus problem, especially as he put another, potentially more damaging problem to rest earlier this year, a stand against the so-called "card check" bill, a measure that makes it easier for workers to unionize with the virtual elimination of the secret ballot. 

This is the central fight of the business community this year, and Specter was not about to find himself on the wrong side of that issue, one senior Senate GOP leadership aide said.

Toomey launched a campaign Web site Wednesday, www.toomeyforsenate.com, with a video announcing his candidacy, and plans to take his message online using the popular social networking Web sites, Facebook and Twitter, Toomey campaign manager Mark Harris told FOX News.

Specter is already using these Web sites, as many politicians follow the wildly successful online ventures of presidential candidate Barack Obama. Toomey also planned a call with bloggers, and a day packed with media interviews, Harris said. A bigger event highlighting his candidacy is planned for May, Harris said.

Political experts in the state note that Specter has alienated the Republican base over the years with his support for abortion rights and gay rights, and other more hard-core conservative issues. And the handful of polls conducted this year bear that out. His approval ratings among Republicans hover dangerously in the 30's. But Madonna told FOX News that about 40 percent of GOP voters are still undecided.

An even bigger hurdle, however, is the approximately 130,000 former Republicans who switched their party registration last April to vote for Democratic presidential candidate Sen. Hillary Clinton, D-N.Y., as well as to vote for Obama in the general election.

"These Republicans are, in the main, concentrated in the eastern part of the state, and many are moderate in their political ideology," Madonna said.

They are a crucial voting block for Specter, who has been traveling the state imploring them to switch back in order to vote in the state's closed primary, something Madonna calls an "unlikely prospect."

But all is not lost for Specter among conservatives, Madonna said. In 2004, the Senator won conservatives over Toomey by 2 percent. 

"Specter is a great campaigner and will make the experience argument. Plus, he has brought billions in federal largess into the state--a plus politically--and in general that has mattered in Pennsylvania," Madonna said.

To be sure, the economic crisis is front and center in this race, and the state of the economy when voters head to the polls will be critical, making this a referendum on Specter's choices more so than about Toomey. If the economy isn't better, Specter has "one killer of a problem," Madonna said.

Madonna noted another factor that could affect the moderate senator. "No one knows how his age, 80 next year, and his health will play out." 

Specter has a form of cancer he has fought twice into remission, something Madonna says proves he is "resilient, tough and like Houdini he is a master escape artist."

Toomey will likely be outspent by Specter, once again. In 2004, the former congressman was outspent by about a 4-1 margin against Specter, who says this year he expects to raise some $30 million for the primary and general elections. 

Specter will also benefit from the support of the national party. But Harris said Toomey has no fear of being outspent. "Pat has a solid message that will sell to Republicans in this state," Harris said.

Specter, a maverick moderate, is often at odds with his party. Although Toomey's fiscal hawk credentials and anti-tax rhetoric align perfectly with the GOP, Pennsylvania is an unpredictable state, trending Democratic with the 2006 ouster of Republican Sen. Rick Santorum, the election of a Democratic governor, and the 2008 election of Obama. 

Specter is trying to capitalize on those trends.

"Mr. Toomey is farther to the right than Sen. Santorum," Nicholas said in Wednesday's statement. "Mr. Toomey's Club for Growth has cannibalized the Republican Party by defeating Sen. Lincoln Chafee, R-R.I., in 2006, depriving Republicans control of the Senate, which denied confirmation to 34 Republican judges."

Specter's Senate leadership is fully behind him.

"While I doubt Arlen could win an election in my home state of Texas, I am certain that I could not get elected in Pennsylvania," Sen. John Cornyn, R-Texas, head of the party's re-election effort, wrote in a letter to Republicans earlier this week. "I believe that Senator Specter is our best bet to keep this Senate seat in the GOP column."

Senate Democrats are likely soon to be just one vote shy of a 60-vote filibuster-proof majority, and Specter is using that as a weapon against Toomey. Specter says Republicans would be handing this critical majority to Democrats if they vote for Toomey in the primary, arguing that the former congressman is unelectable in the general election.

And Cornyn was singing from the same song sheet, telling Republicans, "My job as head of the NRSC is to guide the GOP back to a majority in the Senate. I can't do that without Arlen Specter. With him as our nominee, I can target our campaign resources toward beating Democrats and growing the Senate Republican Conference."

This Cornyn support drew intense fire from radio host Bobby Gunther Walsh on Wednesday.

"I wish John Cornyn would get voted out," he said. "I'm just so fed up with the status quo."

Toomey dismissed the support.

"The leaders of the Republican Senate are elected by their Republican senators," he said. "That's what they do, but the people of Pennsylvania don't decide their votes based on the senator from Texas."

Toomey said he was confident, having won a seat in Congress in a more liberal district, that he could win a general election, and that, in fact, it is Specter who would lose next November. 

"If he wins the primary, he would have a third party candidate for sure," he said. "That would make it impossible for Sen. Specter to win a general election."