The Democratic presidential contest is now between an unstoppable force and an immovable object.

Hillary Clinton is retrenching behind what her advisers call “a demographic brick wall” in Ohio and Texas – believing that Barack Obama’s recent momentum will be brought to an abrupt halt next month by the blue-collar and Latino voters who have largely backed her elsewhere.

Mr Obama still surges forward, putting his faith in the “fierce urgency of now” helping him to vault over the next big round of elections on March 4, when 444 delegates are at stake, in the same way that he has already defied the laws of political campaigning.

Something, or someone, has to give. And eyes are turning to the party leadership of 796 “super-delegates” to be a referee that stops this fight before it reaches the presidential nomination convention in August.

A senior adviser to Nancy Pelosi, the Speaker of the House of Representatives, has suggested that she – along with other “party elders” – will step into the ring if they feel that Democratic hopes of winning back the White House or maintaining control over Congress are being threatened. Ms Pelosi insists that she remains neutral in the race and that her “focus is on reelecting a Democratic majority in the House of Representatives”.

However, her voice would carry great authority among many uncommitted super-delegates on Capitol Hill – and she is said by one of those close to her to be leaning towards Mr Obama. “The party Establishment is not going to turn its back on a candidate who is generating this tremendous excitement and bringing all these new voters into the political process,” an adviser said. Mr Obama’s team is busy pushing the same message, telling members of Congress in districts where he has already won that they would be foolish to alienate their core vote in an election year.

Mrs Clinton still has the edge among super-delegates, not least because Bill Clinton is calling in all the favours he has done them over the past 16 years. Both candidates know that the Democrats are desperate for a win and are putting increasing emphasis on their competing claims to be best-placed to succeed in November’s general election.

The Clinton campaign regards much of current “Obamentum” as media-fuelled hype and says that the picture will look very different after the elections in Ohio and Texas. Her aides profess not to worry that much about Mr Obama sweeping up the February states, with her spokesman, Howard Wolfson, saying that voters have been balancing each other out all the way through this year’s seesaw contest. “Much is made of the concept of momentum but in this primary season it has been precisely the opposite,” he said, “there is no evidence of a stampede one way or another”.

He gruffly dismisses suggestions that relying on future big-state votes make the Clinton campaign resemble that of Rudy Giuliani, the one-time

Republican front-runner, who skipped early contests to concentrate on Florida – only to see his presidential hopes wilt away. Mr Wolfson told The Times that unlike Mr Giuliani, Mrs Clinton had won California and New York. She had “a long track record” of emerging victorious from elections that had been properly contested, he said, without mentioning that her campaign had effectively ceded many smaller states to Mr Obama .

Ohio is regarded not only as big but also natural Clinton country. It is part of the rustbelt and Mrs Clinton is relying on the blue-collar, lower-income vote, who remember the good old days of her husband’s presidency and trust her on issues such as the economy or national security.

In Texas almost a third of the population is of Hispanic origin, a group that skewed heavily towards her in Nevada and California. Many Latinos also say that they owe loyalty to the Clinton name and want a president on the inside track who can deliver for them – rather than an ethnic minority outsider.

Mr Obama’s strategists acknowledge that Mrs Clinton “unquestionably starts out” with significant advantages but they insist that they will run her close or even win one of these states. His aides say that Latino voters have been more sympathetic to Mr Obama’s bid to become the first black president in states such as Arizona and New Mexico, where they are an established community and not competing with African-Americans for low-paid jobs or housing.

In Ohio he is expected to highlight the Clinton Administration’s record on free trade deals, such as Nafta, that are blamed for the loss of many manufacturing jobs.

Aides point out that not only are both states holding “open contests” – allowing the independents who have backed him before to vote – but that he also now has plenty of time, and pots of money, to campaign hard in both states.

“We have demonstrated repeatedly that once people get to know Barack we can come from way behind to either be competitive or win,” one aide said.

There is a more sinister demographic fact that is causing a collective shudder to pass down the Democratic leadership. Mr Obama is consistently trailing Mrs Clinton among white voters and, in the South, white men.

The Clintons would not dare play such a card, even if they wanted to, particularly after the racially charged ructions of South Carolina last month.

The advisers who sneer privately at the fragility of Mr Obama’s coalition of black people and white “latte liberals”, should remember that it was a similar group that elected Mr Clinton in 1992.

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