Specter has announced he will seek re-election as a Democrat.
If Democrat Al Franken prevails in the disputed election in Minnesota, that would give Democrats a filibuster-proof, 60-vote majority in the U.S. Senate.
In a statement, the moderate Specter warned he will not be an "automatic 60th vote" in the Democratic caucus.
The 79-year old Specter has always been a maverick in the GOP and he vowed to continue voting independently as a Democrat. Specter said the Republican Party has "moved far to the right" since his election in 1980. He stressed his political philosophy is now "more in line with Democrats than Republicans."
Specter, a cancer survivor, faced a tough challenge from a conservative opponent in next year's Republican senatorial primary.
Specter said he knows that his decision to change his political alignment has disappointed some of his constituents, but he also said was disappointed by some of the reactions.
"I am unwilling to have my 29-year Senate record judged by the Pennsylvania Republican primary electorate," he said in the statement.
Specter is a former chairman of the powerful Senate Judiciary Committee.
Much now depends on the outcome of the disputed Minnesota Senate race between Democrat Al Franken and Republican Senator Norm Coleman. Coleman continues to file legal challenges over Franken's apparent, razor-thin victory in last year's Senate race in Minnesota.