Sen. Arlen Specter’s (Pa.) defection from the GOP could boost the influence of moderate Senate Democrats, who hope to play a critical role in shaping President Barack Obama’s agenda and could cast the deciding votes to confirm his first Supreme Court nominee.

Specter, a Republican for more than 40 years, announced last week that he is bolting to the Democratic Party. Sen. Tom Carper (Del.) confirmed that he has already invited his Pennsylvania colleague to join the newly formed Moderate Democrats Working Group that he co-chairs with centrist Democratic Sens. Evan Bayh (Ind.) and Blanche Lincoln (Ark.), both of whom face re-election in 2010 in conservative-leaning states.

Centrist Democrats were already looking to play an influential role this Congress, particularly on major legislative items like health care reform and climate change legislation. With Specter now in the fold, and the news that Supreme Court Justice David Souter is leaving the high court, those moderates could prove even more pivotal.

“It’s too early to know just yet. It depends on the issues,— Sen. Mark Pryor (D-Ark.) said. “But it’s very possible that at least on some, [that] scenario is right. At least it’s possible that Democrats will have to work it out inside the caucus to get to 60, and that’s OK.—

Specter’s decision has left Senate Republicans with just 40 seats — one seat shy of being able to muster a filibuster as a Conference. Republican Senate leaders acknowledged that the power to put the brakes on the White House and any of the Senate majority’s more liberal initiatives now rests almost entirely with moderate Democrats.

Always a wild card when he was a Republican, Specter has promised to be equally independent as a Democrat, even though it may result in him rebuffing a popular president who has vowed to do what it takes to secure Specter’s 2010 re-election. Given his conservative views on the judiciary, Specter could align with Republicans and moderate Democrats to influence Obama’s pick to replace Souter.

On issues where Republican Senators are united, or nearly so, passage of a bill is likely to rest with centrist Democrats or those Democrats who — while not necessarily considered moderate — represent GOP-leaning states. That dynamic was on display last week, when 12 Democrats including Specter joined with all 39 Republicans to defeat a bill commonly referred to as “cram down— to address home foreclosures.

Conservative Sen. Jeff Sessions (R-Ala.), away because of a death in the family, most likely would have joined his GOP colleagues in opposing the bill. His vote would have created a coalition of a unified GOP Conference and a dozen moderate Democrats.

Pryor voted against cram-down, as did Carper, who agreed with — and welcomed — the notion that centrist Democrats could be the new center of gravity in the Senate.

“It could be,— Carper said. “I hope so.—

Carper said that, as of late Thursday, he had not heard back from Specter on his invitation to join the centrist coalition.

Over the past decade, both Democratic and GOP minorities have threatened the filibuster with regularity, whether to block a president’s judicial nomination or to stall legislation. Given the regional and political differences moderate Democrats have with their liberal colleagues, the 60-vote threshold for passage of controversial legislation is likely to remain a fact of Senate life.

Specter’s party switch gave the Democrats 59 seats; the installation of Minnesota Democrat Al Franken would give them 60. Franken appears to have narrowly defeated former Sen. Norm Coleman (R) in the delayed conclusion of the November 2008 election, but his installment in the Senate is on hold pending Coleman’s appeal of the results to the Minnesota Supreme Court.

“This could create a perception issue that Democrats automatically have 60 votes,— said one Democratic Senate aide whose boss is considered a moderate. “But filibusters have always tended to be issue-oriented, rather than party-oriented. I don’t know exactly how this changes the game.—