It's the GOP congressional caucus from those states, the districts they're running in aren't interested in amnesties. The goal is to keep the House isn't it or do you want the Boxers and Feinsteins taking that as well? Besides, it's doing what is right for the country and amnesty is not it.
Democrats and liberals actually oppose it. Most Republicans support some sort of reform and it's only the demagogues like Tancredo® that scare off some GOP members.
The fact is, almost no Democrat is credible on the issue of immigration and almost every Republican is, because at the very least Republicans are law-and-order.
And unless the Tancredoites can manage to nominate a challenge candidate who is not -- how should I put this? -- insane, they will look even more fringe than they really are.