Despite polling that should have warned them off against embracing the birth control flap, Republicans have charged full on into electoral catastrophe. But this most recent episode is part of a broader pattern of embracing unpopular positions, despite obvious risks: Medicare, the payroll tax cut debacle, and the debt ceiling mess.

Why are they doing it? Why would a political party choose to risk disaster?

Courting ideological extremism of this sort is definitely a bad idea for a political party. We tend to talk about how the economy determines the results of presidential elections and that most campaign events and candidate traits matter only on the margins. But the political science literature is very clear: The appearance of ideological extremism can be a very serious factor.

The best estimates are that perceptions of a candidate as ideologically extreme will cost him about three points in a presidential election, everything else being equal. That might not seem like much. But a lot of voting is fixed by party identification, so there’s not all much up for grabs. In any remotely close contest, a party really shouldn’t be positioning itself as outside the mainstream.

And yet, they do it — with an apparently sincere belief that it’s going to pay off on Election Day, no matter what anyone thinks. Republicans did this 1964 by nominating Barry Goldwater; Democrats did it in 1972 by nominating George McGovern. Over and over, we see ideological conservatives (and liberals, too) who believe there’s a silent majority out there that will show up for them at the polls if voters are treated to a clear, pure, ideological message. Why?

In part, ideologues are the most likely to fall into information feedback loops, making it easier for them to believe that everyone out there is like them. It’s a rare ideological extremist who realizes that not everyone shares his or her issue positions; what he or she believes all seems so obviously true.

The trick is for parties to pull themselves back from the brink when they stray too far from the mainstream. That can be difficult in the best of times. If the party just won a landslide victory despite drifting too far to the right (or left), as the Republicans probably did in 2010, it can be almost impossible. That’s what seems to be happening now.

Of course, if the economy slips into a deep recession, Republicans will do well in 2012 almost no matter what they say or do. But if the race proves close, what Republicans have been up to this last week is an invitation to disaster.