The battle of words between the Democratic and Republican frontrunners has taken a new twist after Donald Trump accused of Bill Clinton of “sexism” as it emerged the former president would campaign for his wife.

Donald Trump sought to use Hillary Clinton’s own words - indeed, the precise words she had used to criticise him - by suggesting Mr Clinton “demonstrated a penchant for sexism”.

“Hillary Clinton has announced she is letting her husband out to campaign, but he’s demonstrated a penchant for sexism,” Mr Trump said on Twitter. “So inappropriate!”

Mr Trump, 69, who is leading the Republicans in national polls but is some points behind Mrs Clinton in polls measuring a head-to-head between them, used the language she had leveled at him.

In an interview with the Des Moines Register last week, Mrs Clinton, said she deplored the tone of Mr Trump’s campaign.

Her comments followed series of remarks the former tycoon had made, including the suggestion that it had been “disgusting” that she had gone to the lavatory during a break in a recent Democratic debate. He also said she had been “schlonged” during the 2008 campaign.

I will do far more for women than Hillary, and I will keep our country safe, something which she will not be able to do-no strength/stamina!

“I don’t know that he has any boundaries at all. And his bigotry, his bluster, his bullying have become his campaign,” she said.

“It’s not the first time he’s demonstrated a penchant for sexism.”

Mr Trump’s decision to attack Bill Clinton, one of the most natural campaigners of recent American politics, suggests his potential rival is vulnerable on this issue.

Indeed, US media reports have pointed out this is not the first time that the 42nd president has emerged as a potential issue during the 2016 campaign.

U.S. Republican presidential candidate Donald Trump addresses the crowd during a campaign rally in Grand Rapids, Michigan (Reuters)

In 2013, the wife of Senator Rand Paul of Kentucky, said in an interview with Vogue that Mr Clinton's behavior was “predatory, offensive to women” and should preclude him from returning to the White House, even as the husband of the president.

Mr Paul, a Republican, later said he agreed with his wife, but said Mrs Clinton should be judged by her own actions if she ran for president.

Yet, he has continued to argue that Bill Clinton’s presence undermined the Democratic campaign and he used the opportunity to raise the issue of Mr Clinton’ affair with Monica Lewinsky.

“I think really the media seems to have given President Clinton a pass on this,” he said on NBC’s Meet the Press on Sunday.