Defending Main Street, the super PAC of the Republican Main Street Partnership, has gotten $400,000 in donations from two major labor organizations: The Laborers' International Union of North America and the International Union of Operating Engineers, according to the National Journal.

Big Labor mostly donates to Democrats but has been known to send some money to union-friendly Republicans. New Jersey Republican Gov. Chris Christie was able to score some high-profile nominations for his recent re-election bid, for example. But the amount the Republican Main Street Partnership received is higher than usual, the Journal noted:

[T]he scale of these six-figure donations -- $250,000 from the Operating Engineers and $150,000 from LIUNA -- makes this effort distinct. Plus, the money is coming as the Main Street group has been publicly declaring its intent to crush tea-party challengers in Republican primaries, going head to head with conservative bankrollers such as the anti-tax Club for Growth.

"Hopefully, we'll go into eight to 10 races and beat the snot out of them," Main Street President Steven LaTourette told National Journal in October.

That puts these labor groups on the front line of the GOP's intra-party ideological conflict, a battle where the affiliation to unions won't be considered advantageous, to say the least.

IUOE political director Jeffrey Soth characterized the donation as an attempt to support "middle-of-the-road" Republicans. RMSP spokesman Chris Barron said the unions were part of the "wide range of folks who are interested in supporting the governing wing of the Republican Party."