Senator Kerry spent yesterday afternoon meeting with Ralph Nader, an indication that Mr. Nader is emerging as a significant factor in the presidential race. Recent polls show Mr. Nader pulling as much as 6% support. In a close race between Mr. Kerry and President Bush, Mr. Nader’s supporters could be enough to tip the balance.

With Mr. Nader a factor, it’s only natural to wonder whether he will be included in the debates. These debates — September 30, October 8, and October 13 — have already been planned by the Commission on Presidential Debates .

Yet the “selection criteria” for the debates require that to be included in the debate, a candidate “have a level of support of at least 15% of the national electorate” in five national polls. That’s a pretty high threshold — a more reasonable number might be something closer to 5%. The founder of the group Open Debates, George Farah, in his book “No Debate,” quotes Washington Post polling expert Richard Morin as calling the 15% threshold “absurdly high.” Mr. Farah also notes that six weeks before winning the 1998 gubernatorial election in Minnesota, Reform Party candidate Jesse Ventura was polling at 10%.

What’s going on here is an attempt by the two major parties — the Republicans and Democrats — to exclude Mr. Nader and other third-party or independent presidential candidates. The chairmen of the Commission on Presidential Debates are a former chairman of the Republican National Committee and a former chairman of the Democratic National Committee. This may serve the interests of the Republicans and the Democrats, but it doesn’t serve the public. What’s going on is a cycle of exclusion. If a candidate doesn’t hit 15% in the polls, he can’t be in the debate. But if a candidate is denied the national exposure that comes with inclusion in the debates, he is unlikely to reach 15% in the polls.

The objection to including third-party candidates is that they are unlikely actually to be elected president and that their inclusion thus takes away valuable debate time from the genuine contenders. Another objection is that their inclusion might actually boost parties or candidates whose agenda is harmful to America. But such minor candidates as Republicans Gary Bauer and Alan Keyes in 2000 and Democrats Dennis Kucinich and Al Sharpton in 2004 participated in their party primary debates without doing any discernable damage to the republic. The voters were smart enough to decide for themselves that these candidates were not worth nominating. Their presence onstage hardly prevented the more mainstream candidates from getting their message to the voters.

Politics and elections in America are not only about winning candidates but about ideas. Just as the candidacy of Steve Forbes helped get Mr. Bush elected by pulling him to the right on tax cuts, Mr. Nader’s ideas have the potential to filter into the mainstream.

No one can predict how Mr. Nader’s inclusion in the debate would affect the outcome of the presidential race. It’s possible that as more Americans are exposed to him, more of them would vote for him. It’s possible, too, that Mr. Kerry would demolish him in a debate. It’s possible that Mr. Nader’s presence onstage will make Mr. Kerry look more centrist and less liberal — and thus make Mr. Kerry more appealing to swing voters. We’re not arguing for Mr. Nader’s inclusion out of some effort to help Mr. Bush win reelection, or out of some effort to get Mr. Nader elected. We disagree with Mr. Nader on most every issue.

Rather, we’re agreeing with a sentiment of John F. Kennedy, who, Mr. Farah reminds us, said, “A nation that is afraid to let its people judge the truth and falsehood in an open market is a nation that is afraid of its people.”