Sen. Arlen Specter of Pennsylvania has done it again. In today‘s Washington Post, the moderate Republican explains why he bucked the majority of his caucus and backed the president’s stimulus plan.

I won’t waste much space debating the merits of his arguments. The nearly trillion dollars that will be expended on this bill won’t do much to help the economy. It’s just a huge spending bill that will do two things:

On the one hand, the stimulus will enable the president and Congress to pretend they are doing something to help the failed economy. On the other, it gives members of Congress the ability to siphon some of the budget to pet constituencies, a trick at which Specter has himself always excelled.

By jumping on the Obama/stimulus bandwagon Specter has once again been able to position himself as a responsible leader capable of bringing home the bacon for Pennsylvanians — the same formula that has won five terms in an increasingly Democratic state.

Re-election is always on the minds of most residents of the Hill, but on none more so than Specter’s. Though he will turn 80 next year and has struggled with bad health, no one who knows him doubts he will run for re-election. And if he does, Specter would still have to be favored in a general election against most any possible Democratic challenger. Though Dems may feel the rising Democratic tide in the state increases their chances, their best bet was MSNBC host Christ Matthews. But Matthews has already dropped out of the race. Among those still mentioned, none are formidable. A lightweight like Rep. Allyson Schwartz, the hyper-liberal from Montgomery County would be an easy mark for the wily Specter.

But in order to get to that general election in November, Specter will have to retain the Republican nomination and that is far from certain. In 2004, Rep. Pat Toomey, an able and articulate conservative from Allentown, nearly knocked the senator off in a tough primary fight. The only thing that saved Specter, a man most conservative Republicans have always loathed, was the active support of President Bush and then Sen. Rick Santorum, even though Toomey’s stands on the issues were much more in line with their own views. Bush and Santorum both felt that a Toomey victory would mean the loss of the seat for the GOP and possibly the loss of the Republicans’ hold on the Senate.

Five years later, the Republicans have already lost the Senate and have no hope of recovering it anytime soon. Bush is gone and Santorum was himself defeated in a near landslide in 2006. They won’t be able to save Specter from the wrath of conservatives in 2010.

But Toomey might. Last month, the former congressman announced he wouldn’t run for the senate and will focus instead on running for governor of Pennsylvania. He might want Specter to be on the ballot with him in November, even if he can’t stand him. That announcement gave Specter the assurance that he could flout the sensibilities of conservatives once again with impunity.

The question is how Toomey, the current president of the Club for Growth, will react to Specter’s stand on the stimulus. In 2004, no amount of presidential pressure could persuade Toomey to give up his challenge of Specter because he thought it a matter of principle. Is it possible that he will regard Specter’s betrayal of his fellow Republicans on the stimulus to be so egregious that the senator’s defeat will be a greater priority than his own gubernatorial ambitions? That’s not the way politics usually works. But the betting here is that if Toomey did take him on again then the senior senator from the Keystone state would be finished. If so, the price Specter pays for his support for the stimulus would be his chances for a sixth term.