Mr. Obama and Vice President Joe Biden officially welcomed the newest Senate Democrat into the fold during a White House news conference Wednesday.
Mr. Obama said he was "thrilled" at Specter's decision and felt "the American people will be better off for it."
The move gives Democrats 59 Senate seats and they could gain their 60th if Al Franken wins his Minnesota recount (entering its 4th month).
That would give Democrats a filibuster-proof majority that could greatly help Mr. Obama push through his legislative agenda. But Specter repeated his insistence that his vote would not be automatic.
"Anyone who thinks Arlen will cash in his independence have another thing coming," Biden said.
Mr. Obama said he doesn't expect Specter to be a "rubber stamp," but noted their joint interest in health care reform, education and medical research as possible points of cooperation.
On Tuesday, Specter said his decision to leave the GOP came after growing "increasingly at odds with the Republican philosophy."
Some speculate that Pennsylvania's current political landscape would not favor the moderate Republican in next years mid-term election. Specter said he wants a 6th term and "makes no bones about that." (Read analysis about Specter's decision.)
Specter will hope to gain from Mr. Obama's popularity, who has pledged his "full support" for the new Democrat.
Specter said he reached out to his Republican colleagues Tuesday to explain his thinking, but many are rankled by the switch, warning of the dangers of one-party dominance in Congress.
"The threat to the country presented … by this defection really relates to the issue of whether or not in the United States of America our people want the majority party to have whatever it wants without restraint, without a check or balance," said Senate Minority Leader Mitch McConnell, R-Ky.