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As Scott's excellent rundown of tomorrow's Illinois primaries shows, the contest between Reps. Adam Kinzinger, R-Ill, and Don Manzullo, R-Ill., will be a very important test of House Majority Leader Eric Cantor's clout - and, even more importantly, his political judgment.

Cantor took the unusual step of taking sides in the Republican primary contest endorsing Kinzinger, an up-and-coming freshman, over the crusty conservative Manzullo, who has been an occasional thorn in leadership's side. Raising the stakes of the gamble, a super PAC tied to Cantor (YG Action Fund) spent $50,000 worth of radio ads endorsing Kinzinger - in the group's first advertising foray.

The move was designed to promote a young, telegenic congressman whose military background and willingness to be a team player makes him an ideal member, in Cantor's view. But it is looking like it could backfire, big-time. Republican operatives tracking the race said that Manzullo has moved into a narrow five-point lead over the past week, after starting the contest behind. Cantor's endorsement prompted outside conservative groups (FreedomWorks, RedState) to engage in the race on behalf of Manzullo, irked by leadership's bigfooting over a reliably conservative member.

For a leader who touts his relationships with the tea party movement, Cantor's endorsement suggests he doesn't fully grasp the forces animating the conservative grassroots. The 2010 midterms demonstrated the folly of Washington Republicans intervening in intraparty primary fights, with the establishment's favored candidates losing in Nevada, Colorado, Delaware and Kentucky primaries. The NRCC learned its lesson earlier, taking a hands-off approach when its favored candidate in a 2009 New York special election (Dede Scozzafava) imploded, with many Republicans voting for a third-party conservative instead. In all those races, activist antipathy over a political establishment trying to handpick their favored candidates played a major role in their defeats.

A Republican leadership aide said Cantor's public backing of Kinzinger over Manzullo "is raising the hackles of a number of members," and there are worries that leadership is trying to influence the Arizona primary pitting incumbents Ben Quayle and Dave Schweikert against each other.

"A lot of us thought we'd learned our lesson about taking sides in primaries, after that New York race," said the aide. "There has been an understanding not to do so."

Cantor's risky gamble could have serious ramifications. For one, if Manzullo wins re-election (there is no Democratic candidate running in the district), he'll probably be even less of a reliable Republican vote - and fully steamed at his own party's leadership for trying to oust him. Other more-senior Republicans may start worrying they could be falling out of favor, and may be less amenable to being team players.