McCain’s comments could end up having a bigger impact on the prospects of a “comprehensive” bill over the next two years than on the 2008 presidential race. There are internal contradictions in the coalition behind McCain-Kennedy that make the enactment of legislation like it very difficult, despite the support of the White House, the Democratic leadership, and many key Republicans.

One of the divisions is partisan. Republicans who dissent from the Tancredo line still want palpable concessions to pro-enforcement legislators. Democratic amnesty supporters don’t want too much enforcement. Similarly, the GOP likes guest worker programs because it views the issue primarily as a labor supply problem. Democrats, by contrast, prefer a path to citizenship because they want the willing workers to be eager voters as well.

If McCain’s shift is not just a one-off on the stump, this could mean that passing an immigration bill that brings these disparate elements together just got a whole lot more complicated.