WASHINGTON - For years, Vice President Joe Biden has encouraged his friend Sen. Arlen Specter, a moderate Republican, to join the Democratic Party. And for years, it's been largely a one-sided discussion.

Specter finally saw it Biden's way, announcing Tuesday that he was leaving the GOP in a move that put President Barack Obama's party to within one vote of the 60 needed to overcome Senate filibusters.

"You have my full support" and the Democratic Party is "thrilled to have you," Obama told Specter by phone after learning of the senator's intentions when passed a note during a morning Oval Office briefing.

Advisers said that was the first Obama had heard that Specter was switching parties, though they left open the possibility that White House officials, if not Obama himself, knew in recent days that a change was possible.

Democrats inside and outside the White House insisted that Specter made the decision on his own and without pressure from Obama's inner or outer circle. The White House says it didn't promise Specter anything, such as a plum committee assignment or a clear Democratic primary field.

The president, vice president and Specter were to address reporters on the veteran Republican's party switch at the White House on today before Obama makes a trip to Missouri.

Democratic officials privately credited Biden, the former Delaware senator who is close to Specter both personally and professionally, and his persistent prodding through the years with helping create a comfortable, welcoming environment for the five-term Pennsylvania senator to make the change.

"I have been working on that in earnest for the past four years and double time for the past 100 days (as vice president)," Biden told a Democratic fundraiser in Houston on Tuesday.

Specter and Biden served together for many years on the Senate Judiciary Committee and are from neighboring states.

In February, in Philadelphia, Biden thanked Specter for being one of only three Republicans to support Obama's economic stimulus bill and said, "In his heart, I know he's a Democrat."

One senior White House official, who spoke on the condition of anonymity because discussions were private, said Biden and Specter have talked in person or by phone once a week since Obama was sworn into office but switching parties didn't always come up.