Every politician who gets elected to Congress believes that she's going for idealistic reasons. Sure, there are compromises to be made and certain kinds of drudgery to suffer through (particularly fundraising, which they all hate, and justifiably so), but they each believe that they'll do the right thing and work for the kind of change they'd like to see. Nobody gazes up at the Capitol building having been sent there by the people to do the people's work and says, "I'm going to just keep my head down and try not to take any political risks, so I can keep getting elected indefinitely."
But in practice, they frequently face times when they can support something they believe is a good idea for one reason or another, but carries some risk. As comprehensive immigration reform is being considered in the House, each member is going to weighing questions like the following: How much good do I think this bill is going to do? How many votes will supporting it cost me? How hard will it be to convince the constituents who didn't like my support for it to vote for me anyway? Is it going to make fundraising easier or harder? Is the bill going to face a tight vote, so my choice will make a difference? Is my party leadership offering me something to vote the way they want, or threatening to punish me if I don't? And way, way down the list is: How will the outcome of this vote affect my party's long-term prospects in presidential elections?
Some of those questions have easy answers. For instance, Speaker Boehner seems utterly incapable of exercising any influence on anyone in his caucus, so it's unlikely that very many Republicans are all that concerned about what he thinks (and in this case, it's not entirely clear what he thinks, though reports are that like most Republicans who look at the big picture, he realizes it's a bad thing for the GOP if reform dies). As everyone knows, in the last few elections House Republicans have become more conservative, making them less inclined toward passing reform. We often talk about Republican members as being true believers motivated more by ideological dogma than by political considerations, which is true, but only partly because of who they are. It's also because of the districts they come from. Janet Hook of The Wall Street Journal splashes some cold water on CIR's prospects with some simple math:
District by district across the country, there are few House Republicans who have a strong political incentive to support the Senate bill.
An analysis by The Wall Street Journal showed that only 38 of the House's 234 Republicans, or 16%, represent districts in which Latinos account for 20% or more of the population.
In addition, only 28 Republican-held districts are considered even remotely at risk of being contested by a Democratic challenger, according to the nonpartisan Cook Political Report.
She doesn't say what the overlap is between the 28 vulnerable districts and the 38 districts with substantial Latino populations, but the former is probably not completely within the latter, meaning that the number of House Republicans who will be damaged by immigration reform dying is less than a couple dozen.
So to summarize: Within the Republican caucus in the House, there is some subset that believes comprehensive immigration reform is a good idea. Then there's some subset of that group that finds the bill that passed the Senate to be a reasonable approximation of the reform they'd like to see. Then there is some subset of that group that thinks voting for it doesn't bring with it the risk of a primary challenge. When we get down to that subset of a subset of a subset that is actually willing to vote for immigration reform, we're talking about a very small number.