THE CIRCUMSTANCES BEHIND SPECTER’S SWITCH…. Following up on the last two items on Sen. Arlen Specter (R D-Pa.) switching parties, as recently as a month ago, the senator told The Hill, “[Democrats] are trying very hard for the 60th vote. Got to give them credit for trying. But the answer is no.”

Just 10 days later, Senate Majority Leader Harry Reid (D-Nev.) said he would stop trying to pursue Specter as a possible party-switcher.

So, what happened? It doesn’t look as if the White House orchestrated this.

White House aides said on Tuesday that they had no advanced knowledge that Pennsylvania Sen. Arlen Specter would be switching party affiliation from Republican to Democrat. Once told, however, the president reached Specter to express his thrill at having him in the party and to offer his full support.

According to a White House aide, the president found out about the switch at 10:25 AM while in the Oval Office receiving his Economic Daily Briefing.

The president was handed a note, the aide said, that read: “Specter is announcing he is changing parties.”

Seven minutes later, President Obama reached Specter to tell him, according to the aide, “You have my full support” and that we are “thrilled to have you.”

Rather, it seems Specter and Democratic leaders came to agreement: Dems get a 60th vote, Specter gets an easier path to keeping his job. That includes, obviously, skipping a Republican primary he was bound to lose, but it means more than that. Atrios noted this afternoon:

Shuster just said that Dems promised not to field primary candidate against Specter. Obviously they can’t stop someone from running, but it does mean the state party and the DSCC will throw their weight behind Specter to some degree.

George Stephanopoulos added that Specter told President Obama this morning, “I’m a loyal Democrat. I support your agenda.” We’ll see.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.