President says he might not push immigration amnesy this year

Personally, I don't get it. If he thinks it's the right thing to do, and he promised in his campaign to get it done in his first year (a campaign promise already broken), why is he now hesitant to ram it through Congress the way he did with health care reform? From the New York Times:

"We've gone through a very tough year, and I've been working Congress very hard, so I know there may not be an appetite immediately to dive into another controversial issue," Obama said of the immigration issue during a rare encounter with reporters aboard Air Force One as they returned to Washington from an overnight trip to the Midwest.