ROBERT SIEGEL, host:

From NPR News, it's ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris. Today in the US Senate, a political bombshell: Pennsylvania's Arlen Specter, one of the Senate's most senior Republicans, announced he's leaving the GOP and will seek reelection next year as a Democrat. That, along with the prospect of Minnesota Democrat Al Franken winning his state's vacant seat would give Senate Democrats the 60 votes needed to stop Republican filibusters. NPR's David Welna reports.

DAVID WELNA: Word of Senator Specter's leap across the aisle spread like wildfire through the US Capitol today. But when the 79-year-old fifth-term Senator himself confirmed those reports, that electrified atmosphere took on a somber note.

Senator ARLEN SPECTER (Republican, Pennsylvania): This is a painful decision. I know that I'm disappointing many of my friends and colleagues. And frankly, I've been disappointed by some of the responses, so the disappointment runs in both directions.

WELNA: Specter said the Republican Party has moved farther to the right and has left him more in line with the Democratic Party, which he'd left in the early 1960s. But Specter also acknowledged that polls in Pennsylvania showed he had no chance to defeat Pat Toomey in next year's Republican Senate primary. Toomey is the conservative who nearly beat Specter in the party's primary five years ago.

Sen. SPECTER: I have to make a calculation as to whether it's possible, realistic to fight for the moderate wing of the Republican Party in Pennsylvania, and I do not think it's realistic. It's bleak.

WELNA: And so Specter decided last weekend, after repeated entreaties from Senate Majority Leader Harry Reid to go to the welcoming arms of the Democratic Party. Reid was notably understated today in claiming victory.

Senator HARRY REID (Democrat, Nevada; Senate Majority Leader): This is not any time to gloat or give high fives. It's a time to consider a person who took an extremely difficult step to return to the party where he started.

WELNA: At the same time, Reid insisted that Specter - who was one of only three Republicans in Congress to vote for the economic stimulus bill - was pushed out by his party.

Sen. REID: I mean, these people in the Republican Caucus got this right-wing guy to run against him. How is that - does that make you feel pretty welcome in your caucus if you knew people were out there trying to do that?

Senator MITCH McCONNELL (Republican, Kentucky; Senate Minority Leader): This is not a national story. This is a Pennsylvania story.

WELNA: That's the leader of Senate Republicans, Mitch McConnell. He admitted Republicans were not happy with Specter's departure, which the minority leader characterized as purely pragmatic.

Sen. McCONNELL: But it certainly sets up the potential for the majority, if it chooses to, to run roughshod over the majority - over the minority, to eliminate checks and balances and the kind of restraint that Americans have historically wanted from their government.

WELNA: But Missouri Democrat Claire McCaskill said it's doubtful Democrats will always hang together to block Republican filibusters.

Senator CLAIRE McCASKILL (Democrat, Missouri): There will be many issues where some of the moderate Democrats cannot vote with the majority of the Democrats, and I imagine that Senator Specter will join that group.

WELNA: Specter himself left no doubt today about his intentions to vote as he pleases.

Sen. SPECTER: I will not be changing my own personal independence or my own approach to individual issues. I will not be an automatic 60th vote.

WELNA: David Welna, NPR News, the Capitol. Transcript provided by NPR, Copyright NPR.

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