Buoyant Bernie Sanders turns his guns on Donald Trump

Says most Americans and sane Republicans flabbergasted by what's happening

Bernie Sanders will have to fight hard in California, where Hillary Clinton is more than a dozen points ahead (AP)

Bernie Sanders is moving to energise his supporters in Wisconsin, which holds primary elections next Tuesday, by evoking the turmoil in the Republican Party and lambasting its front-runner Donald Trump.

“The American people are just flabbergasted by the nature of this Republican campaign,” he told a standing room-only crowd in the graciously decaying Orpheum Theatre in downtown Madison in the shadow of the state capitol.

“You have incredibly important issues facing the American people and what they are doing is attacking each other’s wives,” he said referring to recent clashes between Mr Trump and Senator Ted Cruz over their spouses. “This is beyond belief. It is something that is not only beyond the comprehension of the average American, but if you are a sane Republican you are also looking at this in disbelief.”

In a sign of growing confidence, Mr Sanders has devoted increasing time to assailing Mr Trump, whom he could theoretically face in the November general election if he pulls off his still long-shot challenge to Ms Clinton for the Democratic nomination.

“Let me remind you, well before he became a candidate, Trump was one of the leaders of the so-called birther movement,” he said, recalling the drive four years ago to question President Barack Obama’s place of birth. “It was a very ugly effort to delegitimise the first African American president in our history. This was not an accident. That was a vicious, divisive attack waged by Donald Trump.”

Nothing if not earnest himself, the self-described democratic socialist, drew repeated whistles and woops of support from a mostly young crowd as he ticked off the main elements of his promised political revolution, from free healthcare and state college education to getting special interest money out of politics and ending oversees wars.

When he spoke of the importance of involving young Americans in the political process, a couple near the front of the orchestra seats fairly threw a baby of surely less than three months into the air.

“As some of you may know, our campaign is on a bit of a roll,” he declared from the stage of the theatre, his thicket of white hair turned purple by the theatre lights. Mr Sanders has won six of the last seven primary and caucus contests. Yet Ms Clinton still holds a formidable lead, especially in the all-important tally of convention delegates.

For that reason, winning Wisconsin next week is critical to the Sanders campaign, which is drawing hope from its from-behind victory in Michigan earlier this month. Though in the end it may be voter turnout that determines the outcome here. A high turnout almost always favours the Vermont senator.

His buoyant mood in Madison may have been fueled by a Marquette University poll released on Wednesday showing him with a 49 per cent to 45 per cent lead over the former first lady. After Wisconsin the circus moves to New York. With a huge trove of delegates, it should be Clinton territory – she was its US Senator twice - but the Sanders campaign believes it can pull off an upset victory there.

Meanwhile, it hopes that by reminding them of the rise of Mr Trump, Democrats will feel impelled to turn out in Wisconsin next week in high numbers.

“I am happy to tell you with absolute certainly that Donald Trump will not become president of the United States,” he told the crowd in Madison. Except, of course, that no one can know that.