Bob Herbert says the Clintons are slinging mud at Barack Obama, and Paul Krugman suggests we should elect people based on substance rather than style. Dream on, Mr. Krugman…Here’s Mr. Herbert:

If Bill and Hillary Clinton were the stars of a reality TV show, it would be a weekly series called “The Connivers.” The Clintons, the most powerful of power couples, are always scheming at something, and they’re good at it.

Their latest project is to contrive ways to knock Barack Obama off his white horse and muddy him up a little. A lot, actually.

Most of the analyses after last week’s dust-up over David Geffen’s comments to Maureen Dowd have focused on whether the Clintons succeeded in tarnishing the junior senator from Illinois. What I found interesting was that no one questioned whether the Clintons would be willing to get down in the muck and start flinging it around. That was a given.

When Senator Obama talks about bringing a new kind of politics to the national scene, he’s talking about something that would differ radically from the relentlessly vicious, sleazy, mendacious politics that have plagued the country throughout the Bush-Clinton years. Whether he can pull that off is an open question. But there’s no doubt the Clintons want to stop him from succeeding.

Senator Obama has come riding out of the wilderness (all right, Chicago) to stand between the Clintons and their dream of returning to the White House and resuming what they will always see as the glory years of the 1990s.

He hurts Senator Clinton in myriad ways. In all the uproar over Mr. Geffen’s comments, hardly anyone has said they were wildly off the mark. There would be no Obama phenomenon if an awful lot of people weren’t fed up with just the sort of mean-spirited, take-no-prisoners politics that the Clintons and the Bush crowd represent. Senator Obama — at least for the time being — is an extremely attractive alternative.

Right behind that as a factor is the distinct possibility that Mr. Obama will ride off with the black vote, without which the Clintons are doomed. Those who joked that Bill Clinton was the first black president are now confronted with someone who might be the real deal.

Senator Obama is also much freer to take fresh stands on the issues. His camp has been delighted, for example, to watch Senator Clinton twist herself into a pretzel on Iraq. From day care to health care to trade and beyond, Mr. Obama is free to offer something new. He’s not tied to the Clinton experience, the Clintonian way of viewing the world.

And, finally, this campaign is not the be-all and end-all for Senator Obama. More easily than the Clintons, he can afford to make mistakes. He does not have to win this election. He can fight another day. In the absence of any catastrophic misstep, he could be selected as a vice-presidential candidate this time around. (It’s not too hard to imagine a John Edwards-Barack Obama pairing.) He can run again for president four years from now, or eight years from now.

His future, as Yogi might have said, is all in front of him.

The Clintons were fresh once. I remember the exhilarating bus tour they took with Al and Tipper Gore right after Bill Clinton won the Democratic presidential nomination in the summer of 1992. There was a spontaneous quality to that tour and a sense that these four young leaders represented a new dawn of American politics.

Almost 15 years later, Hillary Clinton has to fight the perception that she is chasing yesterday’s dawn. She has the benefit of universal name recognition, uniformly high poll numbers and trainloads of campaign cash. But she still gives the impression that she’s riding the political high wire with the mixed blessing of Bill Clinton planted firmly on her shoulders.

It’s ironic that the first woman with a real shot at the presidency comes off not as a compelling underdog but as the powerful front-runner at the controls of a ruthless political machine.

We’ll have to wait and see whether Senator Obama is really offering a new, more hopeful brand of national politics. But here’s a bit of unsolicited advice for a candidate making his first foray into the crucible of presidential politics:

Don’t listen to those who tell you not to fight back against the Clintons. You will not become president if you allow yourself to become their punching bag. Keep in mind the Swift-boating of John Kerry. Raising politics to a higher level does not mean leaving oneself defenseless.

And now here’s Mr. Krugman:

Six years ago a man unsuited both by intellect and by temperament for high office somehow ended up running the country.

How did that happen? First, he got the Republican nomination by locking up the big money early.

Then, he got within chad-and-butterfly range of the White House because the public, enthusiastically encouraged by many in the news media, treated the presidential election like a high school popularity contest. The successful candidate received kid-gloves treatment — and a free pass on the fuzzy math of his policy proposals — because he seemed like a fun guy to hang out with, while the unsuccessful candidate was subjected to sniggering mockery over his clothing and his mannerisms.

Today, with thousands of Americans and tens of thousands of Iraqis dead thanks to presidential folly, with Al Qaeda resurgent and Afghanistan on the brink, you’d think we would have learned a lesson. But the early signs aren’t encouraging.

“Presidential elections are high school writ large, of course,” declared Newsweek’s Howard Fineman last month. Oh, my goodness. But in fairness to Mr. Fineman, he was talking about the almost content-free rivalry between Hillary Clinton and Barack Obama — a rivalry that, at this point, is mainly a struggle over who’s the bigger celebrity and gets to lock up the big donors.

Enough already. Let’s make this election about the issues. Let’s demand that presidential candidates explain what they propose doing about the real problems facing the nation, and judge them by how they respond.

I know the counterargument: you can’t tell in advance what challenges a president may face, so you should vote for the person, not the policy details. But how do you judge the person? Public images can be deeply misleading: remember when Dick Cheney had gravitas? The best way to judge politicians is by how they respond to hard policy questions.

So here are some questions for the Democratic hopefuls. (I’ll talk about the Republicans another time.)

First, what do they propose doing about the health care crisis? All the leading Democratic candidates say they’re for universal care, but only John Edwards has come out with a specific proposal. The others have offered only vague generalities — wonderfully uplifting generalities, in Mr. Obama’s case — with no real substance.

Second, what do they propose doing about the budget deficit? There’s a serious debate within the Democratic Party between deficit hawks, who point out how well the economy did in the Clinton years, and those who, having watched Republicans squander Bill Clinton’s hard-won surplus on tax cuts for the wealthy and a feckless war, would give other things — such as universal health care — higher priority than deficit reduction.

Mr. Edwards has come down on the anti-hawk side. But which side are Mrs. Clinton and Mr. Obama on? I have no idea.

Third, what will candidates do about taxes? Many of the Bush tax cuts are scheduled to expire at the end of 2010. Should they be extended, in whole or in part? And what do candidates propose doing about the alternative minimum tax, which will hit tens of millions of middle-class Americans unless something is done?

Fourth, how do the candidates propose getting America’s position in the world out of the hole the Bush administration has dug? All the Democrats seem to be more or less in favor of withdrawing from Iraq. But what do they think we should do about Al Qaeda’s sanctuary in Pakistan? And what will they do if the lame-duck administration starts bombing Iran?

The point of these questions isn’t to pose an ideological litmus test. The point is, instead, to gauge candidates’ judgment, seriousness and courage. How they answer is as important as what they answer.

I should also say that although today’s column focuses on the Democrats, Republican candidates shouldn’t be let off the hook. In particular, someone needs to make Rudy Giuliani, who seems to have become the Republican front-runner, stop running exclusively on what he did on 9/11.

Over the last six years we’ve witnessed the damage done by a president nominated because he had the big bucks behind him, and elected (sort of) because he came across well on camera. We need to pick the next president on the basis of substance, not image.