A specter is haunting the Republican Party. The specter of marginalization.

Pennsylvania’s Republican Sen. Arlen Specter triggered political shockwaves by announcing he is switching parties, and will run for re-election in 2010 in the Democratic primary.

Specter has evidently decided to cast his lot with the tide of American history rather than with the shriveled puddle of right-wing extremism embodied by the Republican Party.

Maine Republican Olympia Snowe, one of the tiny remaining group of GOP “moderates,” put it this way: ‘We’re heading to having the smallest political tent in history, the way things are unfolding.’

Saying he had “carefully examined public opinion” in his state, Specter declared, “I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate. I have not represented the Republican Party. I have represented the people of Pennsylvania.”

His support for Obama’s economic stimulus drew attacks from Republicans who, deaf to the public mood, foam at the mouth over “government spending” except when the spending helps their fat-cat friends.

A Rasmussen poll showed that as a Republican, Specter was trailing his right-wing primary opponent, Pat Toomey. Pennsylvania is a closed-primary state, meaning that Specter was having to woo a narrow, conservative Republican base. The poll found that “79 percent of Pennsylvania Republicans have a favorable opinion of the ‘Tea Party’ protests against big government spending and higher taxes.”

Since his election in 1980, Specter said, “the Republican Party has moved far to the right. Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats.” He called his political philosophy now “more in line with Democrats than Republicans.”

That may be, but he also said he “will not be an automatic 60th vote for cloture,” and insisted he would not change his opposition to the Employee Free Choice Act.

If Specter wants to be “more in line with Democrats” and with Pennsylvania’s working class majority, he will need to reconsider that stance.

The Democratic sweeps in 2006 and 2008, and especially the election of Barack Obama as president, were propelled by a broad and growing progressive coalition, with the labor movement at its core. It is now fighting to advance a people-before-profits agenda, including expanding worker rights. If Specter wants to avoid being marginalized like his former party, he will need to get on board the people’s ship.