Joe Sestak says even a plea from the president won’t keep him out of the Pennsylvania Democratic primary race against Arlen Specter. And that’s not all:

Sestak also said that he wouldn’t back off even if the major unions reached a deal with Specter on health care and on the Employee Free Choice Act and endorsed Specter in the primary. And in an ironic twist, Sestak also revealed that a few months ago, the Democratic Senatorial Campaign Committee had commissioned a poll testing him running against then-Republican Specter in a general election — and that it showed Sestak winning. Specter, of course, is now the DSCC’s candidate.

Specter is leading in a recent poll but that is close to meaningless over a year before the primary and at a time when Sestak is still not widely known.

Some might think Sestak is being lured into the race by Big Labor and others who simply want Specter to “shape up” and start voting the Democratic party line. But there are two problems with that. First: Specter vote the party line!? Yeah, right. And second: the Democratic primary voters might just like Sestak more. There’s no telling what they’ll think a year from now — after a year of watching Specter and after watching millions of dollars in ads.

What is noteworthy is that at least in some races the prospect of the president and Democratic Beltway establishment campaigning for an opponent isn’t all that intimidating. In fact, by 2010, it may be a net plus.