Proof that the GOP’s terrified of an immigration backlash from the right in November? Or proof that Eric Cantor in particular is terrified of losing his primary?

I’ll accept “both” as an answer.

House Majority Leader Eric Cantor will block attempts next week to include a measure on a must-pass defense policy bill that would legalize young undocumented immigrants who serve in the military.

A spokesman confirmed Friday that the legislation, known as the Enlist Act, will not be among those debated with the National Defense Authorization Act, an annual bill that sets policy for the Pentagon. Rep. Jeff Denham (R-Calif.), the Enlist Act’s chief sponsor, had pledged to bring it up as part of the floor battle over the defense bill.

“No proposed ENLIST amendments to NDAA will be made in order,” a Cantor spokesman, Doug Heye, said in an e-mail Friday. He did not elaborate immediately on the reasons.

Cantor’s always seen legalizing DREAMers as a comparatively easy first step towards a wider amnesty. (He was, and maybe is, writing his own version of DREAM, remember.) They’re the most sympathetic class of illegals who are here; tying permanent residency and ultimately citizenship to their willingness to serve in the U.S. military only makes them that much more sympathetic. Border hawks are leery of it, though — especially if it’s attached to a must-pass measure like the defense bill — because it’s a way to get immigration reform to a conference committee, where Reid and Schumer could dramatically expand Enlist before sending it back to the House.

Both Denham and Buck McKeon claimed that Cantor quietly supported introducing the Enlist Act. The fact that he’s suddenly changed his mind means either (a) he badly misjudged the degree of resistance to Enlist that he’d see from conservatives or (b) he’s genuinely fearful of what’s going to happen in the primary for his seat in Virginia next month. WaPo had a story about that the other day, claiming that challenger Dave Brat was starting to make Cantor sweat with attacks on him over — ta da — immigration. I didn’t take it seriously because it’s so unusual for a big-name House member like Cantor to lose. His 180 on Enlist makes me wonder, though. What numbers is this guy seeing to make him want to pull the plug now, at a moment when Boehner’s signaling his resolve to tackle immigration this summer? If they’re going to move on amnesty, it makes sense that they’d do it soon and also that they might want to start small, with a measure targeting highly sympathetic illegals. Denham’s ready to help on both counts. Instead Cantor’s running away. Hmmmm.

Actually, there’s a third possibility: Maybe Cantor’s not worried about losing his seat or support for the party among righties in the midterms. Maybe what he’s worried about is his caucus. He’s on thin ice with them already; one more screw-up and the dream of Speaker Cantor may be gone for good. While you ponder that, here’s video of him being booed mercilessly by Virginia tea partiers last week.