Republican Senator Arlen Specter as a Republican in January 2007. (Haraz N. Ghanbari/Associated Press)

Senator Arlen Specter's surprise defection to the Democratic Party Tuesday scrambled the political calculus for both parties in next year's Pennsylvania Senate race.

One Democrat quickly decided against seeking the nomination against Specter, the deep-pocketed incumbent who immediately won the backing of the national and state party.

Another who has never held elective office said he would stick it out.

"We are thrilled to welcome Sen. Specter into the Democratic fold and he can count on our full support," Democratic National Committee Chairman Tim Kaine said in a statement.

In a matter of hours, Specter went from a Republican for nearly three decades to Democratic incumbent.

The party switcher has raised about $6 million for his campaign and President Barack Obama has offered to campaign and raise funds for him.

In Pennsylvania, Democrats also have a significant registration edge, 4.4 million to the GOP's 3.2 million.

Republican former Rep. Pat Toomey, the conservative who almost defeated Specter in the 2004 primary, geared up for a path to the Republican nomination that will no longer be the rematch he hoped for.

State Democratic Chairman T.J. Rooney said Specter's decision to run for a sixth term as a Democrat "speaks volumes about where the two parties are, not only in America but particularly in our state."

The GOP requires a "litmus test" to determine whether prospective candidates are conservative enough, Rooney said.

"If they're not pure enough, they're not welcome."