Even as Senator Obama gets set to claim the Democratic nomination for president, the latest Gallup tracking poll shows that for the first time Senator McCain is gaining an edge on the Democratic nominee. It is the slimmest of advantages, to be sure, and one wouldn’t want to read too much into it. Some will note that it is the first reading following his choice of Senator Biden as his running mate. But we’re not so sure that explains it. There is also a growing sense that Mr. Obama is dodging certain questions about his past, most notably in recent weeks the question of his association with a former member of the Weather Underground, William Ayers.

This issue surfaced dramatically this week with a campaign by Mr. Obama to threaten and bully television broadcasters from airing an advertisement that presses the question of Mr. Obama’s relationship with Mr. Ayers, an unrepentant member of the Weather Underground. In 2001, Mr. Ayers told the New York Times: “I don’t regret setting bombs … I feel we didn’t do enough,” referring to the bombings of numerous government buildings, including in 1971 of the very United States Capitol in which Mr. Obama has been going to work in recent years. It may be that the advertisement has errors, as Mr. Obama’s representatives assert. But one of the principles of American law is that opinion is absolutely protected. And when one runs for president, all questions are relevant. The far better way for Mr. Obama to deal with these ads would be to confront them on the substance, the way he did, say, the Reverend Jeremiah Wright.

The ad about William Ayers is being run by something called the American Issues Project and is being funded by what the Associated Press calls a Texas billionaire, Harold Simmons, who, the wire reports say, was one of the individuals who underwrote the work of the Swift Boat Veterans for Truth in their effort to expose Senator Kerry’s war record. It will not be enough, we believe, for Mr. Obama to deal with Mr. Simmons’s allegations by brushing them off as a reprise of the kind of campaign the Swift Boat veterans ran. For many, even the majority, of the Americans who saw the Swift Boat advertisements, Mr. Kerry put up an unconvincing defense, and the Swift Boat veterans performed a real national service.

The allegations about Mr. Obama and William Ayers are no small thing. Mr. Ayers and the Weather Underground represented a dark and violent strain to the movement against the Vietnam war. Not all of those who opposed the Vietnam war were of Mr. Ayers’s ilk. Even those of us who emerged as hawks understand the movement against the Vietnam war had many idealistic and honorable individuals. Senator McCain himself understands that, as he has made clear; he sought reconciliation with the very North Vietnamese regime that held him in bondage for five and a half years.

We also understand that people can change; it may well be that Mr. Ayers has had a genuine change of heart and no longer holds the violent, anti-American, even American-hating views with which he was once associated. But the advertisement and his statements to other publications suggest that Mr. Ayers has yet to apologize for the violent deeds against our country perpetrated by the movement he helped lead. What does Mr. Obama have to say about that? The future senator served with him on the board of a Chicago-based charity as late as December 2002, the Associated Press reports, adding that Mr. Obama “was the first chairman of the Chicago Annenberg Challenge, a school reform group of which Ayers was a founder.”

The AP reported that Mr. Ayers “also held a meet-the-candidate event at his home for Obama when Obama first ran for office in the mid-1990s.” Mr. Obama, according to the AP, “has denounced Ayers’ past activities.” But what of the substance? Not only is Mr. Obama trying to scare television stations from airing the ad but an effort was attempted to prevent the press from gaining access to the records of the Chicago Annenberg Challenge, a point that has been made by Michael Barone. The papers were reportedly released yesterday. America’s voters deserve a straightforward response to the questions being raised, not a campaign by Mr. Obama’s lawyers to threaten television stations that might air the advertisements, which is what, the AP reports, has been happening.

The fact is that the television stations airing the advertisements are carrying out a great tradition, putting before the voters a question that couldn’t be more relevant at a time when America is again in a twilight struggle with a foe who opposes our nation on ideological grounds. Mr. Obama showed, in the case of Rev. Wright, that he can address these questions head on, and even distance himself from a troubling figure. If Mr. Obama was tempted by the hard left and emerged a patriot, so much the better. But if he fails to deal with these questions forthrightly, he will beget only more questions, which is not a logical move at a time when a hero of the Vietnam war is gaining on him in the polls.