Washington ? In the final moments of what may turn out to be the last debate of the Democratic presidential campaign, Barack Obama paid gracious tribute to his opponent, Hillary Clinton.

Invited by NBC’s Brian Williams to specify what questions Clinton still had to answer “to prove her worthiness as the nominee,” Obama said, “I don’t think Senator Clinton has to answer a question as to whether she’s capable of being president or our standard-bearer.” He described her as “qualified” and “capable,” and that “she would be worthy as a nominee.”

Obama was returning the compliment Clinton had paid, when she said at the end of their previous debate that she was honored to share the stage – and this long battle – with him.

The Ohio and Texas primaries are still to come, and if Clinton wins both, the nomination fight will go on. But given what President Clinton has said about his wife having no margin for error in those two states, it is possible that last Tuesday’s debate will be her last.

It has been a strange and remarkable journey, from her early status as the favorite for the prize to this moment of desperate necessity. But as I look back on it, Hillary Clinton has performed impressively. She has nothing to apologize for in her own campaigning, and much of which she can be proud.

A lesser candidate might have cashed in her chips after her humiliating third-place finish in the Iowa caucuses. Instead, Clinton redoubled her workload and pulled out a victory in New Hampshire that astonished even her own staff.

She challenged Obama in South Carolina, where he clearly had the lead, and came back again from that defeat to win California, New York and New Jersey, three of the biggest prizes on Super Tuesday.

After that, Obama got on a roll, while Clinton had to scramble to overhaul her organization and replenish her exhausted treasury, starting with a $5 million personal loan.

But she never quit, and she showed her toughness, not only on the stump and in debates, but in doing something much more difficult – telling her husband to muzzle his personal resentments of Obama and clean up his act.

Substantively, Clinton has more than matched anyone else in the field. As Pennsylvania Gov. Ed Rendell, one of her supporters, told me last week, “Obama has good solid plans, but hers are spectacular” in their detail and sophistication.

Given all that, why is she at risk of being counted out? I think it goes back to what I learned from reading Carl Bernstein’s biography, “A Woman in Charge,” and wrote last September: “This is a complex, talented person who has lived – and survived – a really hard life. As she nears her 60th birthday and the largest challenge of her career, the scars of those earlier experiences are plainly visible. What lies underneath the scar tissue is harder to discern.”

My sense is that in the following months of campaigning, voters were often frustrated by their inability to discover the real person behind the notably buttoned-up candidate.

Her best, and most affecting, moments came when she briefly let her guard down. In early January, in Portsmouth, N.H., when asked how she steeled herself for every day on the trail, she choked up and, with moist eyes, said, “You know, this is very personal for me. It’s not just political. It’s not just public. I see what’s happening, and we have to reverse it.”

There was another such moment at the Texas debate on Feb. 21, when CNN’s Campbell Brown asked her to describe her “moment of crisis.” Clinton said, “Well, I think everybody here knows I’ve lived through some crises and some challenging moments in my life. : (But) with all the challenges I’ve had, they are nothing compared to what I see happening in the lives of Americans every single day.”

She then talked about a visit she and John McCain had paid to seriously wounded veterans at Brooke Army Medical Center in San Antonio, and how moved she was by the courage of those in wheelchairs and on gurneys, the burn victims and the paraplegics. “You know,” she said, “the hits I’ve taken in life are nothing compared to what goes on every single day in the lives of people across our country.”

Those moments allowed people to see the passion and the empathy that have motivated Clinton all these years. But they have been few and far between; unlike Bill Clinton, this is a really private person.

Her inability to break through the scar tissue – the accumulated wounds of a demanding father, a wayward husband and countless political battles – may cost her the chance to be president.