"Arlen Specter is one tough hombre," Obama said during a morning news conference Wednesday inside the White House.
"I have the honor of standing next to the newest Democrat from the state of Pennsylvania. I know the decision Senator Specter made yesterday wasn't easy -- it required long and careful consideration and it required courage. But I know that it also reflects an independence that has been a hallmark of Arlen Specter's career since the days he arrived in Washington. He has never been in the Senate to fight for any particular party, but rather for the men and women of Pennsylvania who sent him here," Obama said.
"I don't expect Arlen to be a rubber stamp," he added.
Specter's switch from Republican to Democrat means that if Al Franken wins in Minnesota, Democrats would have 60 votes in the Senate.
Specter, who appeared alongside the president and vice president, said he will seek a sixth term so he can work on funding for the NIH and medical funds that saved his life.
"I am pleased to run in the primary on the Democratic ticket," Specter said, adding, "I will not be an automatic 60th vote."
On Tuesday, FOX News learned that Biden lobbied Specter to switch to the Democratic Party for six years and has talked to the party-switcher no fewer than 15 times since he voted for President Obama's stimulus bill.
"This has been a long time coming," said one senior official, who spoke to FOX News on condition of anonymity. "He's been having this conversation with Specter for half a dozen years. They're close. You'd be hard-pressed to find a senator he's co-sponsored more bills with."
The vice president and senator are also frequent Amtrak friends, sharing rides from D.C. to Biden's home in Wilmington, Del., and Specter's in Philadelphia. And they've served for years on the Senate Judiciary Committee, where they sometimes teamed up on judicial nominees and crime policy.
"Arlen Specter has been my friend and my confidante," Biden said Wednesday. "Anyone who thinks Arlen will cash in his independence has another thing coming."
Famously, though, Biden and Specter squared off angrily over sexual harassment allegations leveled by Anita Hill against Supreme Court nominee Clarence Thomas with Biden siding, generally, with Hill, and Specter adopting the role of prosecutor trying to punch holes in her testimony.
The official said Biden views the stimulus vote as "a clarifying event" for Specter, one where he decided to back Obama's economic approach and risk retribution from his party. When he faced the prospect of a well-funded conservative challenger, Republican Patrick Toomey, Specter decided toward Obama and the Democratic tilt of Pennsylvania.
More than 200,000 Pennsylvanians switched from the GOP to the Democratic party in 2008 and Obama became the first Democrat to carry the state with more than 51 percent of the vote since Lyndon Johnson.
Specter called Biden's vice presidential office at 8:45 a.m. Tuesday to convey the news, but Biden was in Austin, Texas. The two connected at about 11 a.m.
The official said Biden will follow Obama's lead and, if asked, campaign and raise money for Specter in the primary and general election.
Another senior White House official said Obama was surprised to learn of Specter's decision.
The official said Specter's switch was not the product of a long negotiation with the White House and was not part of a quid pro quo in which Obama officials brokered a deal of any sort.
Even so, advisers swiftly pledged to swing the weight of the White House behind Specter's re-election bid, promising to work on his behalf in a Democratic primary if Specter asks.
"If the president is asked to raise money for Sen. Specter, we're happy to do it," press secretary Robert Gibbs said. "If the president is asked to campaign for Sen. Specter, we'll be happy to do it. As the president told Sen. Specter on the phone, he has our full support."
He added: "I think the president is quite pleased. Understatement of the day."
FOX News' Major Garrett and the Associated Press contributed to this report.