The decision of Sen. Arlen Specter of Pennsylvania to leave the Republican Party after 28 years in the U.S. Senate is obviously part political expediency. Polls have shown that he could lose his seat next year in a Republican primary against former Rep. Pat Toomey.

But the move is also partly a surrender to the liberal impulses that have always blurred his identity and comfort level in the party of Lincoln and, more recently, George W. Bush.

Specter has always been an independent-minded senator who hasn't hesitated to leave the party reservation when his own politics suited him, most recently in supporting President Obama's economic stimulus package. His vote, one of only three from a Republican, was critical in its passage.

Yet prior to jumping ship, Specter had announced to the chagrin of organized labor in Pennsylvania, which had been a strong supporter in the past, that he would vote against the Employee Free Choice Act, the pro-labor bill that would enhance unionizing workforces.

That decision was seen widely as a desperation move by Specter to shore up his Republican base against Toomey, who in a survey by Rasmussen Reports last week led by 51 percent to 30. Specter made a point of saying he would still vote against the pro-labor bill.

Democrats were jubilant at the announcement, particularly because it could give their party the 60 votes in the Senate needed to break any Republican filibuster, provided Democrat Al Franken finally is seated as expected as the junior senator from Wisconsin, in his long-contested court fight with Republican Sen. Norm Coleman.

But Specter demonstrated his unpredictability by saying "my change in party affiliation does not mean that I will be a party-line voter any more for the Democrats than I have been for the Republicans."

"I will not be an automatic 60th vote for cloture," noting his commitment vote against the Employee Free Choice Act.

That decision alone could become a barrier to his re-election, first in the 2010 Democratic primary and, if he survives it, the general election. In any event, as a Democrat he will face heavy pressure from the labor movement and from fellow senators of his new party to change his mind on that vote.

Specter in the past has been an often-cantankerous senator who nevertheless was admired by many Pennsylvanians for his independent streak. In explaining his switch, he took note of both the pragmatism and the philosophy involved.

"Since my election in 1980, as part of the Reagan Big Tent," he said, referring to an open-door policy in the party through which mostly Reagan Democrats passed, "the Republican Party has moved far to the right."

Specter noted that "last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans." There's nothing wrong with Specter's arithmetic, either.

His vote for the Obama stimulus, he said, "caused a schism which made our differences irreconcilable," so he chose not to run again in the GOP primary.

As a member of the Senate Judiciary Committee, Specter may be best remembered for his votes on two contentious Supreme Court Justices' nominations. He voted against Robert Bork, who lost, and for Clarence Thomas, whose winning fight Specter led in the memorable televised hearings in which law professor Anita Hill accused Thomas of sexual harassment.

Specter's party switch, accompanied by a biting slap at the Republican right wing, was not surprising. His decision will put him in Democratic company in the Senate, but he almost certainly will remain a wild card who will take prudent care and feeding by his new colleagues. And he can't rule out a challenger in next year's Democratic primary.

Reach Jules Witcover at (Unknown address).

Reach Jules Witcover at juleswitcover@earthlink.net.