With two short weeks to the Iowa caucuses on January 3, things are getting down and dirty. While mostly keeping their own hands clean, surrogates are throwing buckets of mud in the sure knowledge that slime sticks.

CBS anchor Katie Couric has been asking presidential hopefuls whether voters should trust an adulterer. The way they answered, their directness or lack of it, their body language, and the coy looks on their faces proved a psychologist’s dream.

Senator Clinton’s aides have been mentioning Senator Obama in the same breath as drug dealing and cocaine. Mayor Giuliani, who has been facing awkward questions about his former mistress and his indicted police chief, accused Mitt Romney of employing illegal immigrants. And yesterday, despite denials, the National Enquirer suggested a friend of John Edwards was pregnant with his child.

So Ms. Couric’s questions last night came right on cue. Some answered with a laugh, others with a shrug, some with compassion, others with piety.

But woe betide those who have been keeping secrets. Their denials are sure to return on YouTube to haunt them. Ms. Couric offered a reminder that Presidents Kennedy, Eisenhower, and Franklin Roosevelt had all strayed.

Mr. Edwards gave little sign that he was in the hot seat. “It’s fundamental to how you judge people and human character, whether you keep your word, whether you keep what is your ultimate word, which is that you love your spouse, and you’ll stay with them.”

Mr. Obama knew what Ms. Couric was hinting at. “There are some people who might say that the fact that … I indulged in drugs when I was young disqualifies me,” he said. “Some of our greatest presidents haven’t always been terrific husbands. And some who have been wonderful husbands have been rotten presidents.”

Senator McCain concurred. “I happen to still think that Franklin Delano Roosevelt was an important president at a time in our history when we needed some courage. And so, it’s … a judgment that I leave to others,” he said.

Governor Richardson of New Mexico was clear: “If you’re not faithful to your wife, you’re not faithful to the country.” As was Governor Romney: “I’m certainly faithful to my spouse.” And Senator Biden: “I don’t think people can be dishonest in one aspect of their life, and … be viewed as being honest in other parts of life.”

Senator Thompson of Tennessee offered a general view. “Everybody has weaknesses and has made mistakes one time or another in life,” he said.

It was a line the twice-divorced Mr. Giuliani clung to when he stammered, “I’ve made mistakes in my life. And — and that — not — not just in that area. … I try to learn from them so I don’t repeat them. Sometimes I even repeat them and you — you try again. I mean, you — you — so — I have a — maybe a more generous view of human beings.”

Governor Huckabee of Arkansas understood Mr. Giuliani’s position. “Nobody’s perfect. … We all have flaws.” But he put in the knife all the same. “If you violate the promise that you made to the one person on Earth to whom you’re supposed to be closest to, and this vow was made in front of your families, your closest friends, and God, … then can we trust you to keep a promise that you made to people you don’t even know?”

Senator Clinton knows a thing or two about infidelity. “I think there’s more to someone’s honor and integrity and to their public service,” she said. “I think sometimes we confuse the private and the public in ways that are not necessarily useful. So, of course, it’s a deeply personal matter that I take personally.”

Without naming names, and certainly not her husband’s, she said marital fidelity “would be a tough standard for most of American history to be able to meet, when we look at people who have made a big difference.”

It was a point also made by Mr. Obama. “If we start getting too sanctimonious about some of these issues, then there aren’t going to be that many people who are able or willing to serve,” he said.