THE Democratic presidential candidates had a debate on Sunday in Iowa, and the the pundits were unanimous: Despite attacks by his rivals, Howard Dean’s relentless march toward the nomination wasn’t slowed down at all.

“Dean Still Standing After Foes Take Shots; Old Barbs Do Little to Rattle Front-Runner,” ran the headline on yesterday’s Washington Post analysis by the King of Conventional Wisdom, David Broder.

Hold on a second. True, nobody got off a momentum-stopping debate zinger, like Walter Mondale’s “Where’s the beef?” jibe at Gary Hart in 1984. But there’s every reason to believe Dean was hurt by his performance. His rivals spent a good deal of time bringing up his many strange, ugly and peculiar statements and actions.

It was mostly old news to those of us who follow politics for a living. But with the primaries rapidly approaching, less politically engaged Democratic voters are only now starting to pay close attention to the candidates.

So chances are good that this was the first time many Democrats actually learned that Dean had sealed his records from his days as Vermont governor. Or that he insists we’re no safer since 9/11. Or that he had said he found “interesting” a theory that President Bush had gotten advance warning of 9/11 from the Saudis.

Right now, according to polls, 85 percent of Iowa’s Democratic voters (or, more precisely, caucus-goers) are either supporting a candidate other than Dean or haven’t made up their minds. It’s far from clear that an undecided Democratic voter would come away from Sunday’s debate with a positive feeling about Dean.

He generated a properly scornful laugh for announcing he would balance the federal government in “the sixth or seventh year of my term” – a statement that’s an act of disrespect to voters because it presumes not only that he will be elected in November but reelected in November 2008. He looked baffled and more than a little rattled by the laughter, a deer-in-the-headlights moment from which he did not easily recover.

Worse yet, Dean baldly lied in a way that may come back to haunt him: “You better go look what I said about Saudi Arabians tipping off the president,” Dean fumed to John Kerry, who brought it up. “I said I didn’t believe it, and I said it right on that show.”

Here’s the totality of what Dean said on Diane Rehm’s National Public Radio show on December: “The most interesting theory that I’ve heard so far, which is nothing more than a theory, I can’t – think it can’t be proved – is that he was warned ahead of time by the Saudis. Now, who knows what the real situation is . . .”

Do you see anywhere in this quote an assertion that Dean “didn’t believe it”? (He did use those words on “Fox News Sunday” a week later, after a firestorm erupted on the matter.)

Rarely has a major-party candidate told as blatant (and as easily checkable) an untruth in a debate. By doing so, Dean gave his rivals a new line of attack when they come together again – perhaps at today’s National Public Radio debate, or next Sunday’s televised candidate clash.

The attack he has inadvertently invited won’t come on policy, but on the supposedly straight-talking Dean’s increasing comfort in playing fast and loose with the truth.

Joseph Lieberman was trying to get at that on Sunday when he said Dean was “ducking” his question about sealing his gubernatorial records.

Sealing records is kind of a minor thing. Floating a conspiracy theory that suggests the president of the United States might have been complicit in the mass murder of Americans and then lying about it really isn’t minor. Most Iowa Democratic caucusers don’t like George Bush very much and want to see him defeated, but this sort of thing might give a great many of them pause.

It will probably fall to Lieberman to go after Dean in Iowa for being a liar. Lieberman has already written off the state, so he doesn’t stand to suffer from a backlash there. And he does seem to genuinely believe Dean will do great damage to the Democratic Party and can’t possibly defeat George Bush, so he really doesn’t have anything to lose if the attack is fruitless.

Rueful experience requires me to acknowledge that my prognosticative abilities in presidential primaries have not been flawless. I thought Bill Bradley might defeat Al Gore in 2000 and that John McCain would beat George W. Bush. So skepticism on my assertions here is probably wise.

But I do know what all the rival campaigns know: Howard Dean will walk into the Democratic nomination unless one of the other contenders finds a way to puncture the balloon. Honesty is the issue on which Dean is the most vulnerable right now.

E-mail:podhoretz@nypost.com