Hillary Clinton is campaigning as the underdog in the battle against Barack Obama, pleading with voters to stick with her and telling donors they must help to “level the playing field” before crunch elections on March 4.

In a sweep along the South Texas border region with Mexico — where she cut her teeth as a Democratic activist 36 years ago — Mrs Clinton is being received warmly by the Latino voters who remain her strongest constituency in this state.

“My heart is always in the Rio Grande Valley,” she said at a college campus rally in Brownsville on Wednesday night. Regarded as a dependable ally of the Hispanic community, she got her loudest cheers for promising smart immigration reform that did not divide people from their families.

The Clintons are always at their most formidable with their backs against the wall and they appear to have chosen Texas as the scene for a defiant — possibly last — stand.

Her husband, Bill, has been telling audiences in the state: “If she wins Texas and Ohio I think she will be the nominee.” He said: “If you don’t deliver for her, then I don’t think she can be — it’s all on you.”

Mr Obama is riding the crest of wave after 11 straight victories — including the “overseas primary” of Democrats living abroad whose results were announced yesterday. He continues to draw the largest crowds across Texas, with one rally in Dallas even bursting into applause when he blew his nose, a symptom of the cold that took him off the campaign trail.

He has amassed an enormous cash advantage, allowing him to blanket Texas and Ohio in TV advertising. Figures published showed Mrs Clinton’s campaign would have ended January about $4 million in the red if she had not lent it $5 million of her own money. Mr Obama, by contrast had almost $20 million cash in hand.

Although Mrs Clinton raised $1 million a day in the first half of this month she has been unable to afford the high cost of ground operations in many smaller states and was outspent four to one before Tuesday’s Wisconsin primary. Debts included $2 million owed to her chief strategist, Mark Penn, and unpaid bills for insurance, phone banks, printing and catering.

“We’ve got to even the odds,” she said in an e-mail sent out to supporters. “We must make sure we can hold our own against an avalanche of Obama TV ads, direct mail, phone calls and online advertising.”

Mr Obama is appealing for still more donors so that his total of more than 900,000 contributors will hit seven figures before March 4. His aides were complaining loudly about the efforts of wealthy Clinton supporters to bypass campaign funding laws and create a new group that will pay for attack advertising against him.

Mrs Clinton this week derided Mr Obama’s movement as a “campaign about a campaign”, while recalibrating her message to cast doubt on his credentials to be Commander-in-Chief, handle foreign policy crises and offer solutions for ordinary Americans. Voters face a real choice, she said, and there was only one “candidate who is ready, willing and able to do it”.

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