Sen. Pat Toomey’s endorsement in an August 4 special election for the Pennsylvania state house is raising some eyebrows. That’s because the candidate he’s backing,Paul Mullen, is also the head of the Delaware County AFL-CIO — and business manager of International Brotherhood of Electrical Workers.

What is more, Mullen endorsed Toomey’s Democratic opponent the last time around.

As you might expect, some conservatives are pushing back against Toomey.

In fairness to Toomey, the way it works in a special election is that the party bosses select the nominee — and they selected Mullen. “Pat Toomey supports Republican candidates in general elections,” explained Steve Kelly, a spokesman for Toomey’s campaign.

But the argument that he’s just supporting the Republican is complicated by the fact that a more conservative candidate is running a write-in campaign (always an uphill battle, to be sure). And now, this more conservative candidate, Lisa Esler, and her backers are voicing their concern:

This hasn’t reached crisis level yet, but it’s worth keeping an eye on. The interesting thing is that Toomey, who once railed against Republican leadership as head of the Club for Growth, now finds himself on the other side of the establishment/grassroots divide. What is more, Toomey once found himself battling against the Republican establishment as a conservative candidate for Senate. (Who could forget Rick Santorum’s endorsement of Arlen Specter over Toomey?)

Ultimately, one must view this within the context of Toomey’s imminent re-election battle. And the situation he finds himself in is perhaps a microcosm of the state of the national GOP. He must survive in a hostile environment, which means making compromises to gain voters, while also keeping his base relatively happy. It’s a tough balancing act, and one wonders if he might not have been better off simply staying out of this one.