Democrats have to pander to their base to get elected and Hispanics are a significant and growing part of that base. This forces elected Democrats to aggressively oppose Arizona's attempt to enforce immigration law. This Fox News/Opinion Dynamics Poll (pdf) has found a split among Democrats. Although they are evenly divided in their support for the Arizona law (43% for, 41% against,) 52% of Democrats are willing to leave Arizona alone and see what happens.
FOX News reported:
Significantly more voters think the Obama administration should wait and see how the new law works (64 percent) than think the administration should try to stop it (15 percent).
To varying degrees, majorities of Democrats (52 percent), Republicans (77 percent) and independents (68 percent) think the White House should see how the law works.
The Arizona law makes it a state crime to be in the U.S. illegally and requires immigrants to have proof of their immigration status. Police officers can ask for that proof of anyone they reasonably suspect is an illegal immigrant.