A SESTAK-SPECTER STAND-OFF…. When Arlen Specter announced his party switch last month, he acknowledged he might be trading one primary fight for another. “I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election,” Specter said.

Whether Specter meant it or not, one of those comers will almost certainly be Joe Sestak. Brian Beutler reported last night:

Rep. Joe Sestak (D-PA) appeared on CNN moments ago and confirmed what I first reported earlier today — that he intends to jump into the Pennsylvania Democratic Senate primary against Sen. Arlen Specter (D-PA), pending the blessing of his family.

Sestak has recently begun reaching out to donors in his state, informing them of his plans and asking them to contribute to his campaign in advance of the June 30 FEC filing deadline.

This is not, apparently, an official launch, at least not yet. Sestak continued to emphasize late yesterday that he’ll sit down with his wife and daughter very soon to make sure they’re on board with his plan.

But all indicators clearly point to a big primary fight in the Keystone State. Sestak told CNN he intends to run; he told MSNBC he intends to run; and he told prospective donors he intends to run. If he walks like a Senate candidate, and talks like a Senate candidate….

This is almost certainly a healthy development. Joe Klein made the case for a primary the other day, and I found it very compelling: “Maybe I’m old-fashioned, but it seems to me that if a sitting U.S. Senator decides to change parties simply because he perceives a better chance of winning reelection — that is, if he does so out of zero personal conviction — then he deserves to be primaried simply for form’s sake, to find out what he actually believes.”

Who’s likely to win this primary? It’s fair to say Sestak would enter the contest as the underdog. A recent Research 2000 poll showed Specter leading Sestak among Pennsylvania Dems by 45 points, with the incumbent benefitting from a huge name-recognition advantage.

But I wouldn’t bet against Sestak. He’s proven himself a capable lawmaker; he has an impressive military background; he’s a life-long Dem; he has some money in the bank; and Pennsylvania Democrats are far from sold on Specter, who has given them plenty of reasons lately to question his fealty to his new party.

Should be interesting.

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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.