For most of 90 minutes, Clinton grabbed at every opportunity to accomplish those goals. She questioned Obama's foreign policy credentials. She attacked campaign mailings he had sent out about her as "misleading." She criticized him as failing to reject explicitly the endorsement of his candidacy by Louis Farrakhan, the Nation of Islam leader.

Yet by the end of the night, there was little evidence that Clinton had produced the kind of ground-moving moment she needed that might shift the course of a campaign that polls suggest has been moving inexorably in Obama's direction for weeks.

Instead, in contrast to other debates — where she mixed a warm smile with a sharp attack — she was stern and tense through most of the evening, speaking in an almost fatigued monotone as she recounted her criticisms of Obama, some of them new but many of them familiar. She often sat staring unsmiling at Obama and at Tim Russert of NBC News, who, yet again, presented himself as a tougher challenge to Clinton's credentials than Obama himself.

Her most memorable moment — the one that seemed destined to be replayed in the days ahead — was not, say, a sharp rejoinder to Obama that might undermine his credentials and tilt undecided voters toward her. Rather, it was when she invoked a "Saturday Night Live" skit from last Saturday that showed television journalists fawning over Obama, another example of her campaign's increasing frustration over what it considers unbalanced coverage of the Democratic race.

From the start, Clinton's task going into this debate, broadcast on MSNBC, was far more complicated than the one confronting Obama. National polls as well as surveys in Ohio and Texas suggest that her position is eroding; even former President Bill Clinton said the other day that he could not see Clinton staying in the race if she lost one of those two states. For days, she had zigged and zagged between attacking Obama and celebrating the promise of his candidacy. With a week left, the debate provided what might have been her final opportunity to find an effective line of attack against Obama.

By contrast, Obama's only task was making certain that the campaign did not stray from the road it was on. If he too was low-key and often unsmiling, he sat calm and unruffled, hands crossed, as Russert pressed her again and again. At a point when Clinton apparently saw an opportunity — when she said it was not enough for Obama to simply denounce Farrakhan; he needed to reject his support — Obama did not take the bait.

"I would reject and denounce," he said.

Obama had the advantage of being the candidate with relatively little to prove. The past few weeks have offered increasing evidence that Democratic voters have considered the arguments Clinton and others have made against Obama's candidacy — not ready to lead the country in a time of war; unexamined and subject to an array of attacks by Republicans in the fall; not substantive enough on the issues — and have, for the most part, rejected them.

He was helped by the aggressive questioning of Russert, who made a better case about Clinton's shifting views on trade policy than Obama did. But he also has gotten more self-assured with each debate. And while he did not sit by and take Clinton's attacks — rejecting her assertion that his mailing about her trade and heath care views was dishonest, or that his health care plan would be less effective than hers — he barely bothered to stake out any lines of attack on Clinton, as much a comment on her political strength as anything else he did.

In many ways, he was the foil to her tight and grim demeanor.

Since the debate last week in Texas, and with national polls suggesting that the Democratic party may be rallying around Obama, Clinton had seemed befuddled about what, if anything, she could do to knock Obama off his pedestal. It has been a continued source of frustration for her and her campaign

That did not change Tuesday night in Ohio. Obama, if anything, seemed an even more elusive target. Whether Clinton can unravel this mystery over the next seven days may well determine whether her candidacy will continue beyond the voting in Ohio and Texas.