In an interview with MSNBC on Wednesday, House Budget Committee Chairman Paul Ryan, R-Wis., a leading advocate of reform, described a system in which illegal immigrants could come forward and receive probationary status while — not after — border security work is being done. “…You can be on probation, and you have to satisfy the terms of your probation while the border’s getting secured,” Ryan said.

That — legalization first, followed by completed security — is an entirely different scenario from the one described in the principles. If Republican leaders insist on legalization before security measures are implemented, they’ll likely lose many, many rank-and-file conservative lawmakers.

Of course, the principles give Republicans some room to maneuver. Just what will those “specific enforcement triggers” be? They certainly won’t be a complete security overhaul of the Mexican border. More likely, Republicans will ultimately stipulate that the Border Patrol have complete “situational awareness” — that is, surveillance capability — of the border and also implement interior enforcement measures. But the bottom line is that some work will have to be finished before legalization begins.

At various times in the last few months, it has appeared that immigration reform in the House was dead. Then it seemed to roar back to life. Now, for the first time, the House GOP leadership has committed itself to a set of reform guidelines. Which means the real fight is just beginning.