Politico says immigration reform is a bad issue for both parties:
[T]he polarizing issue is fraught with peril for both parties — so much so that, when asked about the politics of it all, former Republican National Committee Chairman Ed Gillespie paraphrases the words of Democratic New Mexico Gov. Bill Richardson: “When immigration is an issue, nobody wins.”
Of course this is almost literally impossible. It's not completely impossible -- immigration reform could be bad for both parties if it increased the probability of a third party gaining power at the expense of one or both of them -- but the article does not suggest this possibility. Within the two-party context, anything that hurts Democrats electorally helps Republicans, and vice versa.
Now, the catch is that the interests of current elected officials do not necessarily reflect those of the party as a whole. Some of those cross-pressures are regional. For instance, Democrats in heavily white, working class House districts -- and they tend to be the marginal votes in the House -- have every reason to oppose immigration reform. On the other hand, many Republicans might also want to avoid the issue, because it traps them between the desires of their base and swing voters in their own districts.
Probably the more important cross-pressure is time. Arizona Governor Jan Brewer and John McCain are doing massive long-term damage to their party's political viability. But of course they don't care -- Brewer is fighting for her political life (see John Judis's take), and McCain is an old man who won't be around to see the day when a large mobilized Latino bloc would turn the Southwest solidly Democratic. And Democrats like Jason Altmire aren't interested in the future if the democratic coalition, they want to survive 2010.
I think it's pretty clear that immigration reform is great for the Democratic Party. The only problem is that many of the beneficiaries are future office holders who don't get a vote right now.