HERE'S a truth of American politics that at first seems a bit paradoxical. In the National Journal, Ron Brownstein, David Wasserman and Ben Terris write that Republicans in the House don't have to worry about the backlash against the shutdown because compared to a few years ago, gerrymandering has guaranteed that they're now in much safer, more deeply Republican districts. At the same time, as we wrote just after the 2012 elections, gerrymandering helped increase the number of seats won by Republicans such that they retained a solid 33-seat majority in the House despite losing the overall popular congressional vote by 1.4m votes. One might wonder: how can both these things be true? After all, if you're trying to gerrymander electoral districts, you face a choice. You can either cluster all of your voters into fewer districts, which will guarantee that you have very safe districts that the other party cannot win. Or you can spread your voters out more thinly across many districts while ensuring that you gain a majority wherever they are present, which will probably win you more elections but puts each of your members at greater risk. But it's pretty hard to do both at once—concentrate your voters so that each of your members faces little risk of losing, but also maximise your voters' impact so that you win the greatest possible number of elections with the voters you have.

Yet that is what these reports are saying Republicans achieved with the 2010 redistricting, which they largely controlled, since they held most state legislatures. The Republican State Leadership Committee itself boasted that clever GOP redistricting efforts were behind the party's retention of the House last year despite losing the popular vote. Meanwhile, Mr Brownstein et al say they also ensured that GOP congressmen, on several distinct metrics, are in far safer districts than they were during the last government shutdown in 1995. The number of GOP congressmen representing districts won by the Democratic presidential candidate (Bill Clinton in 1992, Barack Obama in 2012) has dropped from 79 to 17. Just 71 Republican congressmen represent districts where Mitt Romney got less than 55% of the vote last year, compared to 141 who were in the equivalent situation in 1995.

Finally, the authoritative Cook Political Report produces a Partisan Voting Index for each district, showing how strongly it trends towards its preferred party. In 1995, according to the National Journal article, the Cook report's average Partisan Voting Index score for Republican-held congressional districts was 6.6. Today, the average is 11.1. Further:

Beyond those averages, the PVI data also show that the share of House Republicans in overwhelmingly safe districts has soared, while the portion in even marginally competitive seats has plummeted. In 1995, 12 House Republicans represented ruby-red districts whose index score leaned toward the GOP by at least 20 points; now 24 represent such districts. In 1995, 25 House Republicans represented districts with a Republican-leaning index score of at least 15; now 61 represent such districts. Conversely, back then, more than two-fifths of the Republican caucus (105 members in all) represented at least somewhat competitive seats with a Republican-leaning index score of 5 points or less. Today only about one-fifth of Republicans (53 in all) represent districts so closely balanced.

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.

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