Kevin Marth stands in the shadow of the vast, closed down Allis Chalmers tractor factory, brown carpenter gloves hanging from his lumberjack shirt, his breath heavy in this freezing blue-collar Wisconsin town. “I’m for Hillary,” he says. “Most people around here are. That Obama guy, I don’t care for him much. I’m afraid of him. He seems like a real slick talker and I just don’t trust him.”

As Wisconsin prepares to hold its primary tomorrow — the next contest in their Democratic nominating battle — Mrs Clinton knows that her fightback now rests in large part on people like Mr Marth: white, working-class voters without university degrees. It is a constituency that has remained steadfastly loyal to the former First Lady and who will have a huge say in three of the next four big contests.

Aides to Mrs Clinton, who only this week began pouring resources into Wisconsin, are playing down expectations. Mr Obama opened field offices here five weeks ago and has been leading in the polls all month.

Yet Mrs Clinton is quietly hopeful that voters such as Mr Marth will allow her to beat expectations in Wisconsin — or even pull off a win. The polls have tightened in recent days with Mr Obama only four points ahead. Even a close result will yet again change the narrative of their see-saw contest after Mr Obama’s eight straight post-Super Tuesday victories. While Mrs Clinton’s aides have been talking about the undeniably vital importance to her campaign of the March 4 contests in Texas and Ohio, a decision was made last week to take on Mr Obama in Wisconsin.

There are fears in the Clinton camp that another big win for the Illinois senator tomorrow could make his momentum unstoppable — and Texas and Ohio too late — in a race that has now come down to wooing the 300 unelected super-delegates who remain uncommitted to either candidate.

Because the race has been so close, it is virtually impossible for Mrs Clinton or Mr Obama to win enough elected state delegates to get the 2,025 needed to clinch the nomination, even after the final contest in Puerto Rico on June 7.

This now makes the elite club of the party’s 795 super-delegates — its members of Congress, former Presidents and top officials — the potentially decisive electorate in the race. They can back whoever they like. Both campaigns now view the 300 who remain uncommitted as their most critical audience.

Wisconsin, Ohio and Texas are vital to Mrs Clinton if she is to convince this tiny but enormously powerful group of her viability, especially as several black members of Congress previously pledged to her switched to Mr Obama last week.

If she loses all three states, she may well lose the super-delegate race — and thus the nomination. Mr Obama is expected to win the other contest tomorrow, the caucuses in Hawaii, the state where he grew up.

Mrs Clinton spent millions of dollars on television advertisements in Wisconsin last week, criticising Mr Obama for failing to debate with her here. With a new campaign manager, Maggie Williams, she also unveiled a sharper, economically populist message. Like Wisconsin, Ohio and Pennsylvania — which votes on April 22 — are economically challenged states with a big proportion of white, blue-collar voters. She is also questioning the substance of Mr Obama’s grandiloquence. “Speeches don’t put food on the table,” she is telling rallies.

Yet Wisconsin also reveals the weaknesses and tactical shortcomings of Mrs Clinton’s campaign. With its white, blue-collar demographics — half the Democratic electorate earns less than $50,000 (£25,000) a year — it ought to have been fertile territory for her, but there are no Clinton campaign signs in West Allis.

The mixed messages her camp has sent over Wisconsin have puzzled many strategists. Mrs Clinton arrived in the state only on Saturday and leaves today. Her campaign never expected the race to last this long and is now trying frantically to play catch-up with Mr Obama’s consistently superior ground organisation.

“Obama’s out-hussled us,” said Jerry Rintamaki, a Wisconsin plasterer supporting Mrs Clinton, after a Bill Clinton rally in Milwaukee.

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