SPECTER’S BOLD QUESTIONS FOR SESTAK…. With Rep. Joe Sestak poised to take on Sen. Arlen Specter in a Democratic primary in Pennsylvania next year, it stands to reason that Specter is going to take some rhetorical shots at his rival. I just didn’t think this would be one of them.

Pennsylvania Sen. Arlen Specter called his fellow Democrat, Rep. Joe Sestak, a “flagrant hypocrite” and accused his rival of registering as a Democrat “just in time to run for Congress.” […]

On Thursday, Specter’s campaign sought to bring into question Sestak’s roots to the Democratic Party. Specter’s campaign sent out a list of Sestak’s voting history in Delaware County, which the senator’s campaign said showed that Sestak registered as an Independent in 1971, didn’t vote in any primary elections from 1971-2005 and that he officially registered as a Democrat in February of 2006. Sestak was elected as a Democrat to the House in 2006.

“Congressman Sestak is a flagrant hypocrite in challenging my being a real Democrat when he did not register as a Democrat until 2006 just in time to run for Congress,” Specter said in the statement. “His lame excuse for avoiding party affiliation, because he was in the service, is undercut by his documented disinterest in the political process.”

Sestak, who routinely describes Specter as a “flight risk” for the Democratic Party, didn’t hesitate to issue a forceful response. Soon after Specter’s criticism, the House member explained that he was a registered independent throughout his 35-year career in the Navy — which is why he never voted in a primary — because he believes “military officers should be nonpartisan.” After retiring from active duty, Sestak said, he registered as a Dem.

As for Sestak’s “documented disinterest in the political process,” he added that there were several elections in which he voted with absentee ballots, which he said are routinely discounted for service members “because they arrive too late.”

But those details aside, the fascinating aspect to this is that Arlen Specter is questioning someone else’s party loyalty. Given the circumstances, that takes some real chutzpah. Sure, Sestak wasn’t officially a member of the Democratic Party until three years ago, but Specter was a long-time Republican until three months ago.

Isn’t this a subject he’d want to avoid?

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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.