The Wall Street Journal editorial board warns that the immigration debate threatens to make the GOP a minority party. They're right. It splits the Republicans right down the middle, demoralizes the base in advance of 2008, and is prompting a conservative counter-mobilization that could make Latinos a Democratic constituency for years to come.
Ironically, the issue was not pushed to the top of the legislative agenda by Democrats. As John B. Judis points out, Democrats have struggled to push through legislation that splits Republicans and forces a Bush veto (for now, at least, GOP party loyalty is generally too strong to overcome a filibuster).
Instead, Bush has been doing the Democrats' work for them. In 2005-2006, he tried to push the unpopular concept of Social Security private accounts, which scared off GOP moderates and eventually died. Now he's pushing an awkward immigration compromise that alienates conservatives without exciting anyone besides David Broder.
The combination of Bush's lame duck status and the configuration of the House, Senate, and presidency means that it's very difficult to pass important new legislation and keep your coalition intact. The President is finding this out at great political cost.
Correction 6/28 6:58 AM: In this Congress, Pelosi and Reid have passed two bills that Bush was forced to veto (an Iraq war funding bill and stem cell legislation), not zero. This error is corrected above.