Sen. Arlen Specter was expected to face a tough primary challenge in 2010.
A Specter party switch would give Democrats a filibuster-proof Senate majority of 60 seats if Al Franken holds his current lead in the disputed Minnesota Senate race.
Specter, a five-term Senate veteran, was expected to face a very tough primary challenge in 2010 from former Rep. Pat Toomey, who nearly defeated Specter in the Pennsylvania GOP Senate primary in 2004.
Numerous Republicans are angry with Specter over his recent vote in support of President Obama's $787 billion stimulus plan.
Specter was one of only three GOP senators who voted for the measure.