Hillary Clinton and Barack Obama this morning acknowledged that tonight's Super Tuesday results would settle little in a Democratic presidential race that threatens to grind on into next month and beyond.

Before voting near her home in New York - one of 24 states staging 43 Democratic and Republican contests in today's unprecedented "national primary" - Mrs Clinton said: "We're all kind of guessing about what it's all going to mean because it's never happened before."

Her rival, Mr Obama, said: "The fact that we've made so much progress I think indicates that we've got the right message, and the question is are we going to be able to pull some states out. No matter what happens though, we're probably going to see a split decision tonight."

In an electrifying battle that will see either a black man or white woman as a US presidential candidate, the pair headed into the 22-state Democratic contest with Mr Obama having pulled even with Mrs Clinton, or even slightly ahead. He has enjoyed a fortnight of extraordinary momentum, wiping out her double-digit leads in national polls in critical states such as California, New Jersey and Missouri.

A month after their early contests amid the snow and ice of Iowa and New Hampshire - with Mr Obama winning the former and Mrs Clinton launching a stunning comeback in the Granite State - voters speak today coast-to-coast, from frozen mountains in the north to the red earth of the Deep South. There are elections in Alaska and Alabama, New Mexico and New Jersey, Idaho and Oklahoma.

Yet, because Democratic Party rules stipulate that the 1,681 delegates up for grabs today are awarded proportionately - a candidate can lose a state but return a similar number of delegates - both campaigns expect the two rivals to roughly split the delegates on offer.

It makes for the closest Democratic race in a generation, and one that may not be decided until Ohio and Texas vote on March 4, or even when Pennsylvanians got to the polls on April 22 - a prospect of deep dread for already exhausted aides and strategists. To win, one candidate must reach the magic number of 2,025 delegates.

Yesterday both sides were engaged in a game of expectation management. "The nominating rules of our party are really designed to prolong a contest between two strong candidates," said Mrs Clinton's spokesman, Howard Wolfson. "Many of us will be making our reservations for Texas and Ohio, and perhaps Pennsylvania and beyond that."

Mr Obama's campaign manager, David Plouffe, was equally cautious in his predictions. "We fully expect Senator Clinton to earn more delegates on February 5th and also to win more states," he said in a memo to reporters. If Obama wins a few and stays within 100 delegates of Clinton today, he said, "we will have met our threshold for success."

The Republican elections may provide more clarity. Of the 21 states holding contests, eight award delegates on a winner-takes-all basis. This makes it easier for one candidate to build up a big lead tonight when there will be 1,023 delegates up for grabs.

Mitt Romney boomeranged across the country and back in a 37-hour dash, branding himself the true Ronald Reagan conservative at every stop.

"I think you're going to see a growing crescendo of Republican conservatives getting behind my candidacy," the former Massachusetts governor said, before conceding: "Right now that hasn't entirely happened."

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