The White House and senior Democrats have grown increasingly nervous that Sen. Arlen Specter, who switched parties last year, will lose his Democratic primary to Rep. Joe Sestak. A senior political strategist who is consulted by the White House puts the odds at "50/50." 

President Obama is expected to campaign for Specter late next week or just before the May 18 primaries. Sestak has insinuated that White House chief of staff Rahm Emanuel tried to force him out of the race by dangling a top-level Defense Department job.

Specter's troubles aren't terribly surprising. Rank and file Democrats remain frustrated at their party and are skeptical of hand-picked candidates. Though Specter has moved to the left since his party switch, voters know that he was a Republican for 30 years. Early polling in the race was almost certainly inflated because Specter was a known commodity statewide, and few people outside of Sestak's district had heard of him. That's slowly changed, and Sestak has crept up on Specter in the polls.

Today, Sestak went for the kill, releasing a television ad showing Sarah Palin and George W. Bush praising Specter -- a political taunt that irritates Democratic trigger points. Specter still holds the line in polls. He is popular with independent leaning Democrats, who tend to be less liberal, and with African American voters, who have always supported him more than they tend to support other Republicans.