ARUNNING joke in political circles is that Sen. Arlen Specter is actually wearing a GOP face mask to cover up his Democratic heart.

In a meeting billed as the Middle Class Task Force held in Philadelphia last week, Vice President Joe Biden encouraged Specter to change his party affiliation to Democrat. But Specter says his GOP voice is essential to getting things accomplished in Washington, even if it troubles his decades-long political career.

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I wasn't surprised by Biden's suggestion that Specter change parties because I'd asked Specter the same question a few weeks ago at a meeting with a group of African-American media managers, reporters and editors who questioned him intensely about the $787 billion stimulus package.

He said he doesn't hesitate to buck his party in order to vote for the right issues. Pennsylvania's senior senator said he's a committed Republican, but says he'll do the right thing in Washington, even if it means risking his political career, which his pro-stimulus vote may already have done. The reports of candidates eagerly lining up to challenge him are proof of that.

But Specter argues that voting yes on the president's stimulus was essential to preserving the nation's financial stability and future. And as one of only three GOP senators (along with Susan Collins and Olympia Snowe of Maine) to vote for the plan, Specter was key to guaranteeing there wouldn't be a fatal Senate filibuster blocking the bill.

Specter says he knew before ink was dry on the stimulus bill that he'd put his job on the line. A new poll by Susquehanna Polling and Research indicates than half of the 700 voters who were questioned want to replace him.

But Specter insists that more important to him than winning re-election is to help get the country back on track, and he may have to cross party lines again and incur the wrath of colleagues and constituents in the process. Some of his colleagues have already called Specter's decision "politically perilous."

The point of the president's stimulus package was to identify "shovel-ready projects" to help create millions of jobs. But Specter told the president he had reservations about the speed of the operation. Washington's critical mistake with the bank bailout money, the $700 billion TARP (Troubled Asset Relief Program), was that the bill moved too quickly to put congressional oversight in place before the first half of the money was doled out. Specter says that was a disgrace and will move to make sure that it doesn't happen with the second $350 billion.

But he may face an even more challenging battle on Capitol Hill. Facing his sixth run for the Senate, Specter is mum on which way he'll vote on pending legislation called Employee Free Choice Act that will make it easier for workers to form unions.

But Specter's a survivor, or he wouldn't have remained on Capitol Hill for 29 years. It's still too early to predict what will happen, but with the 2010 elections just over the horizon, challengers are ready to take aim.

Political speculation says viable candidates include Democrats Joe Torsella, former CEO of the Constitution Center, as well as U.S. Reps. Allyson Schwartz and Bucks County's Patrick Murphy.

Although Specter's GOP constituency is clearly perturbed at his more liberal views, it's not yet clear who from that party might rise up to give him a strong challenge in the primary.

No matter. Specter says he's feeling great. He argues that cancer couldn't keep him out of Washington politics and that he still plays an aggressive game of squash daily - both signs that he's still a strong fighter.

Meanwhile, his war chest continues to grow. According to the Federal Elections Commission, he already has $5.8 million in his arsenal. At age 79, and even with many GOPers miffed at him, Specter says don't ever count him out or be surprised when he speaks as his party's voice of reason.

Just ask some of his colleagues, who already know better than to try to twist his arm to vote along party lines when he disagrees in principle. *

Fatimah Ali is a journalist, media consultant and an associate member of the Daily News editorial board.