Sen. Arlen Specter (Pa.) has consistently been watching the conservative flank of his party over his shoulder since almost losing the GOP primary to then-Rep. Pat Toomey in 2004.

But on Monday, the possible peril he faces in a 2010 intraparty battle was staring him in the face.

Toomey said in a statement that he is reconsidering a run for Senate in 2010 because of the federal government’s recent “corporate bailouts— and “unprecedented spending and debt.—

“I think we are on a dangerously wrong path,— Toomey said. “Pennsylvanians want a U.S. Senator focused on real and sustainable job creation that gets our economy growing again.—

Specter defeated Toomey, who is now president of the Club for Growth, by two points in 2004. He would be conservative Republicans’ first choice again to face Specter in a primary, according to national GOP consultant John Brabender, who works for several clients in Pennsylvania.

“I think clearly that Toomey would be an extremely formidable candidate for the U.S. Senate and against Specter,— Brabender said. “I think absolutely that race would be in play immediately. I think if the election were held today, I would not be surprised if Toomey would win.—

Toomey had been mulling a run for either Senate or governor in 2010, when Gov. Ed Rendell (D) is term-limited out of office. But he announced in January that he was opening an exploratory committee for a gubernatorial run, which ended talk about a potential Senate rematch — until Monday.

Specter’s crucial vote for President Barack Obama’s $787 billion stimulus package was hailed by national Democrats, but panned by conservatives in his home state — a sentiment that he addressed in his comments after his vote on Feb. 13.

“My vote was cast recognizing the very substantial political peril that I face,— Specter said. “I know that there are many on the Republican political spectrum who do not like the vote. I remember, obviously, the tough primary fight I had in the year 2004. But I felt in the final analysis, given the very severe consequences which might befall the country, that my duty was to look out for the public interest and not my own personal political interest. That is why I voted as I did.—

What’s more, Democrats have consistently siphoned off Republican voter registration in the Keystone State over the past six years. The result is a more conservative GOP electorate than last time Specter faced re-election in a closed GOP primary.

But the worst might be yet to come. Specter supported cloture on the Democratic-backed Employee Free Choice Act. Should he vote the same way when the legislation comes up again this Congress, the sweeping labor reform act would go to the floor of the Senate and would likely pass. Specter has not said whether he plans to vote for cloture again for the legislation or if he supports the bill.

All of these things, however, continue to make Specter more popular with Democrats than with his own party in Pennsylvania — a situation that has been consistent throughout his political career, according to Pennsylvania Democratic strategist Larry Ceisler.

“He’s always had problems with the right wing of his party because be doesn’t walk in lock step with anyone, let alone the right,— Ceisler said. “He’s kind of a one-man party, to certain degree.—

Ceisler, for one, said he plans to vote for Specter in the general election in 2010.

Democrats, meanwhile, must prep for any possible electoral scenario in 2010. If Specter wins his primary, he’ll be tough to beat in the general election. But Ceisler said Toomey can’t win a Senate general election.

“He needs to figure out what would give him the greatest satisfaction, defeating Arlen Specter in the primary or being governor of Pennsylvania,— Ceisler said of Toomey.

National Democrats acknowledge that Pennsylvania, unlike other battleground states, has a deep bench of candidates who have expressed a strong interest in running.

Prominent Democrats, including Reps. Patrick Murphy, Joe Sestak and Allyson Schwartz as well as state Auditor Jack Wagner and state Rep. Josh Shapiro, have been mentioned as potential candidates looking at running for the Democratic nomination. But none seem to be moving toward running at this point.

Former National Constitution Center President Joe Torsella, who lost the 13th district primary to Schwartz in 2004, is the only announced Democrat in the Senate race. Torsella and Specter worked closely together on the newly constructed $230 million center in Philadelphia, and the two have a friendly relationship — even hiring each other’s wives.

“Sen. Specter and I are both running for our parties’ nomination — whether or not we’re running against each other is a question for another day,— Torsella said in a Monday phone interview. “My fundamental belief is that the state is ready for change and that we’re better served with a Democrat in the seat.—

Torsella, who worked directly under Rendell when he was mayor of Philadelphia, has also hired the governor’s top consultant, Neil Oxman, to do his media for his campaign. Torsella, however, was coy about whether a Rendell endorsement was forthcoming.

And if Toomey decides against a Senate bid after all, a couple of other Republicans are still looking at the race. Pittsburgh venture capitalist Glen Meakem has expressed interest in the Senate seat, even going as far as issuing a statement blasting Specter for his vote for the stimulus bill.

“I consider Arlen Specter a friend, but his support of this bill is too much,— Meakem said after the vote. “There will be a Republican primary fight for Specter’s Senate seat in 2010, and I am going to be actively involved in electing someone who will do what’s right for Pennsylvania taxpayers, not the Washington lobbyists.—

Keystone State sources say Meakem would likely step aside for Toomey but could mount a well-financed bid if the former Congressman does not run.

Anti-abortion activist Peg Luksik also confirmed that she is seriously considering running against Specter. Luksik ran for governor twice, once in the 1990 GOP primary and again as an Independent in 1999.

“At this point, I’m very close,— Luksik said in a Monday phone interview. “Probably by the end of the month there’ll be an announcement.—

And unlike Meakem, Luksik is more likely to run in the primary with both Toomey and Specter and could split the opposition vote so the Senator squeaks out a victory.