The White House photographer Pete Souza captured the moment when Senator Arlen Specter told President Obama that he was switching parties:

Interestingly, we hear Specter tried to tell Vice President Joe Biden first. For years, Biden had been trying to convince Specter that he should become a Democrat. The two are close; they had sat alongside each other on the Senate Judiciary Committee, and shared many Amtrak rides. As it happens, however, Biden was in Texas when Specter called his West Wing office at 8:45 this morning, so Obama was the one who got the word. Here's the timeline we get from the White House:

10:25 AM - While in the Oval Office and during his Economic Daily
Briefing, the President was handed a note that said "Specter is
announcing he is changing parties."

10:32 AM - President Obama reaches Senator Specter and tells him "you
have my full support" and that we are "thrilled to have you."