Usually Charlie Cook’s smarter than this (bolding mine).
Every time Obama takes a public stand on immigration, he makes it that much more difficult for Republican members of Congress to support it. Keep in mind that 94 percent of House Republicans are in districts Mitt Romney carried and that 34 of 45 GOP senators represent states Obama lost. As a result, most congressional Republicans are far more afraid of losing a primary to a more conservative challenger than a general election to a Democrat. It is a lot easier for them to support an immigration bill that has broad-based support in the business and farming communities (and that also happens to be supported by Obama and the Democratic leadership) than to back a bill so popularly identified with the other side. If the president really cares about enacting immigration reform, he will get off the campaign trail, depoliticize it, and keep as quiet about it as he can.
Via Ramesh Ponnuru, who is just aware as I am that the President doesn’t really care about enacting immigration reform. Particularly since nobody at the moment can argue with a straight face that the GOP isn’t having an, ah, open and frank internal exchange of opinions on the entire issue.