But primaries poison the health of that system and warp its natural balance, because the vast majority of Americans don’t typically vote in primaries. Instead, it is the “third of the third” most to the right or most to the left who come out to vote — the 10 percent at each of the two extremes of the political spectrum. Making things worse, in most states, laws prohibit independents — who are not registered with either party and who make up a growing proportion of the electorate — from voting in primaries at all.

The phenomenon of primaries’ pulling people to the extremes seems more prevalent in the Republican Party, where centrists and moderates are increasingly rare, as a result of a combination of factors since the 1970s — the shift of Southern states toward Republican control, the mobilization of evangelical voters around social issues, anti-tax movements in California and elsewhere, and the rise of conservative talk radio and other news media. But the dynamic could easily expand to include the Democrats, who have at times been pulled too far to the left, for example on issues like crime and welfare’s excesses in the 1980s.

Two additional factors exacerbate the problem of party primaries. The first are the deep-pocketed interests that often lie at the extremes. The loosening of campaign finance restrictions by the Supreme Court has unleashed a flood of “independent” political spending by these special interests.

The second is the redistricting process. Technology has allowed parties that dominate their state legislatures to draw districts that will almost never elect a candidate of the opposing party. Each party maneuvers, once a decade, to manipulate the boundaries to its advantage. One of the reasons the Senate is, for all its flaws, still more moderate than the House is that there is no redistricting, since senators are elected statewide.

Primary election rules are not immutably ingrained in our politics. Before the McGovern-Fraser Commission — formed after the Democratic convention in Chicago in 1968, which was marred by conflict over the Vietnam War — primaries were not even a major component of electoral politics in most states. Both parties adopted many of the commission’s recommendations, which were intended to weaken the power of party bosses. But today, with the decline of party-machine power, the polarization that divides the parties seems a far greater threat than establishment “bosses.”