A significant factor in the polarization of American politics is the “closed primary” — contests that restrict participation to registered party members. This is an unanticipated consequence of what was meant to be a positive reform of the Democratic Party nominating system, preventing strategically mischievous “crossover voting.” As Democrats changed state statutes to close their primaries, the laws generally affected Republican primaries as well. By excluding independent voters, who generally are ideological moderates, the restrictions narrowed the internal debate within both parties and accelerated the radicalization of American politics.

I am not a bystander to this process. I have served as executive director of the Democratic National Committee and as a member of three Democratic delegate selection reform commissions that incrementally moved to restrict participation in our party nominating processes. We changed our rules in 1972 to prevent more incidents like that year’s Michigan Democratic primary, at which Republicans voted in large numbers for Alabama Gov. George Wallace. Our intent was strategic and honorable: Why should Republicans be allowed to vote to nominate a racist, unelectable Democrat?

We also wanted to make the process more representative of the party. So from 1972 to 1992, Democrats replaced a large number of state caucuses with primaries. Republican state parties closed a number of their primaries as well and significantly increased the number of primaries over caucuses. There were 16 state Democratic presidential primaries when the reforms took effect in 1972. We expect that there will be 37 next year. Almost 70 percent of these will be closed to participation of other-party supporters and independents. In 2012, the Republican Party will have primaries in 42 states, 25 of which will be closed, including delegate-rich Florida, Pennsylvania, Michigan and New York.

Closed primaries affect politics and public policy. They are empirically skewed to the parties’ base constituencies, exaggerating their role and impact. They produce spectacles such as the one we saw during the recent debt-ceiling crisis, when the ideological rigidity among House Republican freshmen prevented Speaker John Boehner from negotiating a “grand deal” with President Obama that included a balance of revenue enhancers and entitlement reform — an approach supported by two-thirds of Americans, according to an August Gallup survey.

Incumbents from states with closed primaries have reason to be sensitive to inter-party challenges if they stray from the base ideology. These days, it is the only political threat that seems to resonate with some. One of the major stories of the 2010 elections was successful internal party challenges to Republican moderates by conservatives: Utah Sen. Bob Bennett was bested by Mike Lee in the GOP primary in May 2010. Christine O’Donnell’s primary win over Rep. Mike Castle almost certainly resulted in Democrat Chris Coons winning the Delaware Senate seat. Republican and Democratic moderates who used to align in cross-party coalitions often feel constrained by primary threats. The resulting ideological radicalization of Congress and strict “party voting” results in more than increased confrontation and less consensus: We have a Congress of ideologues representing a country of pragmatists who increasingly feel alienated by partisan rancor. Indeed, a Gallup poll found that on the debt-ceiling debate, 82 percent of Americans thought members of Congress were guided by partisan advantage while only 14 percent thought they were acting out of what was good for the country.

The impact of closed primaries will be felt in campaigns of all level next year. Setting aside House and Senate contests, consider that the Republican presidential nominating process will begin with the closed, ideologically unrepresentative caucus in Iowa, then move to New Hampshire’s more inclusive primaries — in which registered independents can participate — before shifting to a closed caucus in Nevada.

The fourth contest, the Feb. 28 primary in South Carolina, is potentially decisive. Earlier this year the state’s Republican Party raised a legal challenge to the traditional open primary, seeking to limit participation to registered Republicans. Although a federal judge ruled that the primary must remain open, state Republicans are appealing, and pointing out that courts have generally permitted parties to regulate their own internal processes. In 2000, of course, moderate Republicans and independents flocked to Sen. John McCain in the South Carolina primary, almost derailing George W. Bush’s campaign. If South Carolina Republicans prevail in closing their primary, Texas Gov. Rick Perry is likely to have a big advantage over seemingly more moderate Mitt Romney.

Parties can, of course, do what they wish. And if closed primaries reflect the views of the people who represent the breadth of the party, then there is nothing wrong with the contests. But so far closed primaries have led to candidates catering to extreme views — and the red meat that is so tasty to primary voters has not proved digestible in general election contests. It would not take much for registered party members and ideological moderates who are registered to vote to have the ability to choose between reasonable people who seek to be legislators, not gladiators.

The hybrid model used in New Hampshire allows registered independents, who often determine general election outcomes, to participate in the Democratic or Republican primary while protecting each major party from crossover by the other. If expanded to all states, such a system could moderate American politics: It would produce candidates who would fare better in general election contests and elected officials who are more flexible and willing to compromise across party lines. This simple change could help restore civility to our politics, perhaps creating room for bipartisan solutions to our country’s great problems, and revive faith in the American political system.

The writer, a partner in Locke Lord Strategies, was executive director of the Democratic National Committee from 1974 to 1977.