The Hill's Aaron Blake has the story.

Former Rep. Pat Toomey (R-Pa.) will challenge Sen. Arlen Specter in a primary for the second straight cycle, according to Pennsylvania GOP sources.

The Allentown Morning Call reported Thursday that two friends of Toomey's have said the Club for Growth chief has decided to enter the race.

I have already written about this race a couple times this week, but to synthesize in light of this latest development, it's hard to see a simple way out of this situation for Arlen Specter. The veteran Republican Senator has to realize that regardless of how far to the right he moves at this juncture he's not going to win a Republican primary against Pat Toomey, whom he only very narrowly beat in 2004 in significantly more favorable circumstances (the Bush White House and then-Senator Rick Santorum backing him up, but also the Republicans holding just a 2-seat majority in the Senate and worries that the chamber could flip back to the Democrats).

From this vantage, the only way Specter wins another term (and it's hard for me to envisioning him not wanting this) is if he leaves the Republican Party (though not necessarily the Republican caucus) and calls himself and independent for 2010. In a three-way race against Toomey and a Democrat, Specter wouldn't have to carry a majority of Republican primary voters, or indeed even a majority of Republicans overall. Instead, he would only have to pull in perhaps a third or so of Republicans, as well as a good chunk of independents and Democrats (which he has fairly consistently done). In such a scenario, Specter would be freed up to vote on measures like the Employee Free Choice Act, among others, as he would be competing not only for the votes of conservatives but also the votes of moderates and liberals.

Specter could always opt to retire, but again, I don't really see that happening. How he gets out of this pickle -- if he can -- should be quite an endeavor to watch.