Gone are the days of conservative Democrats and liberal Republicans. Now the most liberal Republican in the Senate is more conservative than the most conservative Democrat. This ideological sorting has been reinforced by gerrymandered House districts that leave fewer truly competitive general-election districts.

These factors affect not just elections but also how the House and Senate do business. For most members in both chambers, the most important election is the primary race, and primary voters are reluctant to reward compromise and eager to reward ideological adherence. The pressures that force candidates out to the ideological extremes in elections also discourage bipartisanship. The same factors leading to an increasingly polarized electorate are creating an increasingly polarized Congress.

Note that Snowe did not say she was leaving the Republican Party because it has become too extreme. Nor did she say she was being forced out. She is not leaving the GOP; she’s leaving the Senate. And poisonous partisanship is a bipartisan problem.