March 4 was not supposed to be a “Super Tuesday.” But, as voters head to the polls today In Texas, Ohio, Vermont and Rhode Island, this could be the definitional day of the race for the 2008 Democratic presidential nomination.

Just as the first Super Tuesday, on February 5, was once expected to “seal the deal” for Hillary Clinton, so this Super Tuesday, could “seal the deal” for Barack Obama.

Will it happen?

Bill Clinton said at the start of the fight for two states with big Democratic National Convention delegations Ohio (141 pledged delegates) and Texas (193 pledged delegates) that Hillary Clinton had to win both if she intended to stay in the race.

But on the eve of the voting, Hillary Clinton said, “I intend to do as well as I can on Tuesday and we’ll see what happens after that.”Of course, Obama backers will try to apply Bill Clinton’s high and narrow standard tonight.

Clinton backers will stick to Hillary Clinton’s low and wide standard.

In an ever-evolving race, which has been all about setting, abandoning and resetting expectations, definitions of victory and defeat are fluid.

So how will the expectations game actually be played?

Here are some basics:

If Obama wins both the Ohio primary and the Texas primacaucus (the strange hybrid of a daytime primary and a nighttime caucus that will play out in the Lone Star state today) — and, for good measure, primaries in Rhode Island (21 pledged delegates) and Vermont (15 pledged delegates) – the Illinois senator will not just own Tuesday’s bragging rights. He will be able to stake an honest claim on the nomination.

Even without all the delegates that he needs, Obama will establish both a clear lead and insurmountable momentum.

Anything short of a big-state sweep – ideally with some small-state advantages – will make things more complicated for the senator.

Is that fair to Obama? Probably not.

Just a few weeks ago, Obama was trailing Clinton in Ohio, Texas and Rhode Island. And, not all that long ago, Dennis Kucinich might have given the senator from Illinois a run for his money in liberal Vermont.

Now, he needs a whole lot of wins to win.

With front-runner status you get front-runner expectations.

If Obama wants to force Clinton out of the race this week – and avoid the long, hard fight until the late April Pennsylvania primary, a May Oregon primary or perhaps the battles for Montana and South Dakota in June – he has to meet or exceed them.

That’s why Obama has been spending, spinning and campaigning so hard since the February 19 Wisconsin primary.

After breaking even on February 5 and then winning the next 11 primaries and caucuses, he can win it all today. But to do so, he must finish the day with a definitional “win.”

What’s that?

A four-state sweep would be definitional. Winning Ohio, Texas, Rhode Island and Vermont would close the deal. And Clinton, even though she has a good deal of the money in the bank, would almost certainly “fly to New York to huddle with aids” – code for preparing to suspend or end her candidacy on Wednesday or Thursday.

Ohio, Texas and Vermont wins would do the job. It is tough to imagine how even the best political spin machine could make the case that from Rhode Island, the nation’s smallest and most Democratic state, can a rebound be launched.

But what if there is a big-state split?

What if Ohio goes for Clinton and Texas for Obama, or vice versa?More likely than not, the race goes on. Theoretically, Obama wins in Texas, Vermont and Rhode Island and a very close result in Ohio might be enough to knock Clinton out.

But anything that looks like an unsettled finish keeps Clinton going. She has the money – her fund-raising operation is working well, collecting roughly $1 million a day — she’s better on the stump, her campaign getting better at getting under Obama’s skin. None of this means that Clinton is doing better than Obama. But that’s not the point. No one folds a campaign that can point to wins and that has the money to carry on.

Of course, the quality of the wins she can point to will matter.

For Clinton, obviously, the best scenario is a solid Ohio win and any kind of Texas victory — even a split decision that gives her the primary and the Obama the caucuses and an overall delegate advantage. Toss in Rhode Island and she owns the day, bragging rights and something she’s been missing for the better part of a month: momentum.

Anything less is life-support rather than real revitalization. But a campaign on life-support is still a campaign. And it can still make Obama sweat – for at least six more weeks – just as it can give Republican John McCain something even better than the expected exit of Mike Huckabee: an opportunity to look presidential while two battling Democrats continue to look for one another’s soft spots.