The only Jewish Republican in the Senate has defected to the Democratic Party, causing a potentially major shift in the senatorial balance of power.
Pennsylvania’s Arlen Specter, one of the longest serving senators, has long been at odds with his Republican colleagues.
His moderate views on economic and social issues became increasingly unwelcome and he publicly disputed the Republican caucus’s decision to oppose President Barack Obama’s economic recovery plan.
“This is a painful decision. I know I am disappointing many of my friends and colleagues. Frankly… the disappointment runs in both directions,” Mr Specter said in a Capitol Hill news conference on Tuesday.
While stressing the growing ideological rift between him and his former political home, Mr Specter also acknowledged the practical reasons that led him to switch sides.
He was facing a tough primary race in the state of Pennsylvania against a conservative Republican leading in the polls with a wide margin.
Crossing to the Democratic side gives Mr Specter a better chance of keeping his seat.
The decision shocked Washington not only because of the symbolic value of having a senior Republican abandon the party, but also because of the shift it could bring about in Senate.
With Mr Specter’s defection, the Senate Democrats now have 59 members, only one senator shy of the 60 needed to block any Republican filibuster attempts.