Republican presidential front-runner Donald Trump has predicted the US is headed for a “very massive recession”.

He has warned that a combination of high unemployment and an overvalued stock market had set the stage for another economic slump.

• Trump backtracks on punishing illegal abortions
• Paris attacks suspect to be extradited
• One dead in ‘all out blitz of gunfire’

“I think we’re sitting on an economic bubble. A financial bubble,” the billionaire businessman said in an interview with The Washington Post published on Saturday.

Coming off a tough week on the campaign trail in which he made a series of missteps, Trump’s latest comments bring him back into the limelight ahead of Tuesday’s important primary in Wisconsin where he trails in the polls.

The former reality TV star said that the real US jobless figure is much higher than five per cent number released by the US Bureau of Labor Statistics.

“We’re not at five per cent unemployment,” Trump said.

“We’re at a number that’s probably into the twenties if you look at the real number,” he said, adding that the official jobless figure is “statistically devised to make politicians – and in particular presidents – look good”.

Trump trails Ted Cruz in Wisconsin. Getty

Trump said “it’s a terrible time right now” to invest in the stock market, offering a more bleak view of the US economy than that held by many mainstream economists.

The interview was bylined by the Post’s Robert Costa and Watergate reporter Bob Woodward.

Trump vowed in the interview to wipe out the more than $US19 trillion ($A24.78 trillion) national debt “over a period of eight years”, helped by a renegotiation of trade deals.

“I’m renegotiating all of our deals, the big trade deals that we’re doing so badly on,” he said.

After making controversial statements about abortion last week, Trump has shown little sign of heeding calls from fellow Republicans to adopt a more presidential tone so as to avoid alienating voters in the November general election if he wins the nomination.

On Saturday, he questioned close US ties to Saudi Arabia and again accused US allies of not pulling their weight in the NATO military alliance.

Trump told a campaign rally in Racine, Wisconsin that partners in the North Atlantic Treaty Organisation “are not paying their fair share” and called the 28-nation alliance “obsolete”.

Tuesday’s Wisconsin nominating contest could be a turning point in the Republican race. Trump, 69, trails his leading rival, US Senator Ted Cruz, 45, of Texas in the state.

A Cruz win would make it harder for Trump to reach the number of 1237 delegates needed to secure the nomination before the Republican national convention in July. The winner will get to claim all of Wisconsin’s 42 delegates.