After 19 debates between eight Democratic presidential candidates, then seven, then six, then four, then two, it is easy enough to think that there is nothing left to be said by the remaining contenders: Illinois Senate Barack Obama and New York Senator Hillary Clinton.

In fact, what is said during tonight’s debate at Cleveland State University is likely to matter more than what was said at all the debates that came before it.

No matter what words are used, no matter how messages are delivered, no matter who gives the evil eye or the warm pat on the back to who, it will have meaning.

That’s because the 20th debate could be the last in this marathon race for the Democratic nomination.

There is no question that Obama is surging. He has won 11 primary and caucus contests since Super Tuesday, the last voting day on which Clinton or her increasingly dispirited campaign team could claim anything akin to bragging rights. Obama now leads in commitments from pledged delegates – those chosen by the voters – and is rapidly closing the gap among uncommitted “super-delegates.” He has a 3-1 fund-raising advantage. He has just secured many of the most coveted union endorsements – from the Service Employees, the Teamsters and their Change-to-Win coalition – and his new backers are already spending freely on his behalf. Newspaper editorial pages in Ohio and Texas – which will hold critical primaries on March 4 – are busily outlining reasons to vote for the Illinoisan. And it is certainly reasonable to suggest that Obama is positioned to secure the majority of delegates chosen on March 4; he’s pulling close or ahead in Ohio and Texas and even if he were to lose by narrow margins in those states, he’s likely to win very big in at least one other state that will be voting that day, Vermont. Then there are the new national polls that show the Illinois senator opening a wide lead over Clinton among Democrats – 54-38 in the fresh New York Times/CBS News survey, with a tie among women and a better than 2-1 advantage among men – and running stronger than his rival in mock-ups of a fall contest with Republican John McCain.

And what of Clinton?

Husband Bill, who has proven to be about as useful to Hillary Clinton prospects in 2008 as former Florida Congressman Mark Foley was to House Republicans in 2006, has as much as said that he can’t see his wife continuing as a serious contestant if she fails the Ohio and Texas tests. If that wasn’t bad enough, the Clinton campaign is veering wildly from conciliatory language, like the candidate’s “I am honored to be here with Barack Obama” close of last week’s debate in Austin, and body blows, like Clinton’s Saturday suggestion that the Obama camp is borrowing tactics from arch-Republican Karl Rove.

For Obama, who has been less than enthusiastic about debating of late, tonight’s forum is not without its perils. The revered political speaker of the moment is still only an O.K. debater.

Though he has had a few good performances, he has yet to actually “win” a Democratic debate. And he has lost a few. At once long-winded and vague, and frequently sharper in his barbs than the candidate’s nice-guy image suggests possible – as when he edgily pronounced Clinton “likeable enough” in a New Hampshire debate — Obama has yet to master the forensic competition that may matter most in a fall fight with McCain.

Still, Obama does not need to win anything tonight. Moderators, questioners, pundits and viewers remain inclined to cut the senator slack. And they will probably keep doing so until right around the time that he moves from frontrunner status to that of presumptive nominee. If Obama simply holds his own, as he did admirably last week, he’ll keep rising in the polls.

The Clinton corollary to all this is, of course, that if Barack only must hold his own, Hillary absolutely must prevail. We’re not talking about winning “on points” here. Clinton needs a knock out. And she needs it not on some “change you can Xerox” cheap shot, like the one that failed her so miserably in the Austin debate, but on a substantive point of difference between the campaigns.

The Clinton camp has in recent days been preaching a “Get Real” gospel that continues, in somewhat gentler form, the suggestion of Bill Clinton that there is a fairytale aspect to the Obama run. That’s not exactly a winning line. A winning line would clearly connect Obama’s failure to support a mortgage-foreclosure freeze to his support from the banking industry, or dramatize the discussion of what Obama’s refusal to commit to universal health care reform means for real people are really sick.

The problem for Clinton, of course, is that her campaign is awash in corporate contributions and her health care plan is universally flawed.

So Clinton is in a corner, with few options. She has already teared up. She has already gone gracious. In recent days, she has besmirched Obama and played at being amused by him. None of it has worked very well – while the moist eyes in New Hampshire may have yielded transitory benefits, nothing of the win in that state has proven to be sustainable – and the time clock is running out.

The stakes have never been higher. Clinton will win tonight, or she will almost certainly lose the nomination to Obama. The distractions are gone. The excuses have run out. This debate matters because there will only be another one if Hillary Clinton has figured out how to change the shape of things to come.