When Arlen Specter avowed himself a Democrat last week, tossing a wave and a slip-of-the-tongue endorsement for Norm Coleman over his shoulder as he left the GOP, it wasn’t the first time he had left one party for brighter prospects in another.

A 1965 New York Times article titled “Upset in Philadelphia” recounted Specter’s overnight switcheroo from registered Democrat to Republican winner of the Pennsylvania Attorney General contest, “after having failed in a bid for the Democratic nomination.” This time, Specter saw the nomination battle coming, and he ducked.

During his 29 years as a Republican senator, Specter held office through the Reagan years and the high tide of the Republican party before watching 200,000 Pennsylvania Republicans change their voter registration to Democrat.

Somewhere in those decades, he became the Democrat’s Republican, the man who said that his party shouldn’t be “captive to the demands of the intolerant right” and took Ralph Reed and Pat Buchanan to task for inserting their religious conservatism into politics.

Specter doesn’t look the maverick part; he’s serious, some might say stiff. His voting record is one of calculated centrism. For every vote with the Republican establishment for freezes on discretionary spending and mandated spending cuts in times of growth, there’s an “aye” for such Democratic positions as limiting immunity for telecoms who assist in wiretaps and expanded stem cell research.

He placed himself firmly at odds with the GOP this spring when he supported Obama’s stimulus package, following up his “yes” vote with support for an extension of the Children’s Health Insurance Program and the Democrat-driven Lily Ledbetter Fair Pay Act.

But if Specter angered quite a few Republicans in 29 years, he may have angered more Democrats in the first week as their new and most junior colleague. Observe Senate Majority Leader Harry Reid on CNN grit his teeth as he says that, “We’re doing our best to make him happy as a Democrat,” adding, “We think he is.”

If so, Specter has an unorthodox way of showing his pleasure at being a Democrat. Since leaving the Republican party, he has wavered out loud over the Employee Free Choice Act, voiced his opposition public insurance provisions of Obama’s health care plan, and voted no on an amendment that would have given bankruptcy judges more flexibility to alter mortgages for homeowners in trouble. True, he had warned that he was “not an automatic 60^th vote.”

All this has the progressive grassroots peeved at what some see as inside baseball between Specter and the D.C. political establishment.

The idea that Democratic party leaders have ruled out a 2010 primary challenge to Specter prompted Adam Green, co-founder of the Progressive Change Campaign Committee, to launch an online “straw poll” asking voters to weigh in on whether Rep. Joe Sestak should be encouraged to take on Specter in the Pennsylvania primary.

“Right now it’s purely in the realm of political insiders,” said Green. “We wanted to give the grassroots a voice in whether there should be a Democratic primary … if it is worth spending the time, energy and money on a primary campaign. If it’s 50-50 [for a primary], that tells us something. If it’s 90-10, then we can get to work.”

Green observed that in the short term, having Specter as a Democrat has benefits. Since, he said, Specter seems to “lack core convictions,” peer pressure from Democrats is preferable to peer pressure from Republicans.

Andrew Perez, a rising junior at George Washington University, set up his own online petition, primaryarlenspecter.com, focusing on Specter’s opposition to universal health care legislation.

“I think anyone who calls himself a Democrat should be trying to pass universal health care,” he said.

Both Green and Perez are cautious about the prospects for a progressive candidate. A far-left-leaning Democrat may not play well across Pennsylvania, and Joe Biden remains in good enough physical shape to tackle any unwanted contenders.

And if there’s anything Specter has proven himself able at, it’s being a savvy political flexitarian. At least until 2010, odds are he’ll continue to keep both parties guessing no matter which caucus he’s sitting on.

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