In the redistricting that happened after the 2010 census - and the 2010 election where Republicans did very very well which allowed them to control a large number of state legislatures - a large districts were drawn where Republicans dominated, but not too much. Republican votes were spread enough to guarantee majorities in a large enough number of districts to ensure control of legislature despite having fewer overall votes than Democrats.
They guaranteed themselves a majority, with a maximum number of safe seats. This makes them immune to negative opinion about things like shutting government down.
It was a neat trick:
Republicans have managed to both make their seats safer, and ensure there are more of them, despite the fact that they lost the overall popular congressional vote. How did they do that?
By finding the golden mean. The ideal strategy for elections is to make sure your districts have just enough of a partisan tilt to ensure you'll almost certainly win them, but not so much that you win them overwhelmingly and waste your votes. Meanwhile, you want to cram the opposition's voters into districts which they win by overwhelming margins and thus waste their votes. Republicans can make sure their seats are both safer and more numerous by achieving lots of districts where they're likely to win by a safe but not extravagant margin, say 15-30%. If they pursue this strategy, they should wind up with relatively fewer seats that tilt overwhelmingly Republican. Meanwhile for Democrats, whose votes have been "cracked" or "packed" such that they lose more districts, the districts that they do hold would be more likely to be overwhelmingly Democratic than is the case for Republicans.
And this is what the Republicans' redistricting appear to have achieved. Of members of congress who won their districts with a margin of 60% or more in 2012, 18 were Republicans, while 29 were Democrats. In the crucial safe-but-not-overwhelming zone, with victory margins between 15% and 30%, Republicans won 92 seats while Democrats won 42. The average margin of victory for Republicans was 28.6%; for Democrats, it was 35.7%.
So there you go. This is one big reason why Republicans in the House are likely to react to widespread anger over the shutdown by becoming more, rather than less, confrontational. The vote distributions in their districts would have to swing by 10% or more for any sizable number of them to lose their seats, and that's not very likely to happen.
