Democratic presidential candidate Senator Bernie Sanders. Picture: Patrick Semansky/AP/African News Agency (ANA)

Milwaukee - US President Donald Trump threw his support, welcome or not, behind Democratic presidential contender Bernie Sanders in his back-and-forth with rival Elizabeth Warren, saying he did not believe Sanders would suggest a woman could not win the 2020 election.

Tensions between Warren and Sanders, two progressive US senators who are running for the Democratic nomination, have risen after a report that Sanders told Warren in 2018 that he did not believe a woman could beat Trump.

Warren confirmed the report late on Monday in a statement, saying: "I thought a woman could win; he disagreed."

Sanders has denied making such a statement.

Trump, at a rally in Wisconsin on Tuesday, sided with the self-described democratic socialist.

"I don’t believe that Bernie said that. I really don’t. It’s not the kind of a thing he would say,” Trump said, adding that he did not know Sanders and did not like him.

Trump beat former Secretary of State Hillary Clinton in the 2016 White House race.

Trump may have reason to favor Sanders in the Democratic primary race. At rallies, his promise to prevent America from becoming a socialist country always get loud applause. If Sanders were to win the nomination, Trump would be able to make that argument a key and popular part of his re-election pitch.

With the Democratic presidential candidates debating in Iowa on Tuesday night, Trump's rally in Milwaukee served as a sort of counter-programming event.

Trump won Wisconsin by about 23 000 votes in 2016, shocking Democrats, who are pushing hard to recapture the state in 2020 and staging their nominating convention here in July.

Trump wants to keep the state, and Republicans have put significant resources toward making that happen. Trump and Vice President Mike Pence are focusing particular attention on Wisconsin, Michigan and Pennsylvania in their quest to stay in the White House.