WASHINGTON (Reuters) - Republican presidential front-runner Donald Trump waded into politically risky territory this week when he accused Democrat Hillary Clinton of exploiting her gender to win votes and said she would have little support if she were not a woman.

As Mr Trump and Mrs Clinton, fresh off big wins in north-eastern state primaries on Tuesday (April 26), circle each other for a potential matchup in the Nov 8 United States presidential election, his comments portended what could be a nasty campaign.

"I think the only card she has is the women's card," Mr Trump said in a victory in New York on Tuesday. "She has got nothing else going. Frankly, if Hillary Clinton were a man, I don't think she would get 5 per cent of the vote. And the beautiful thing is, women don't like her."

On Wednesday on a programme on TV channel CNN, he continued: "She is a woman," he said. "She is playing the woman card left and right. She didn't play it last time with Obama. But she's playing it much harder this time and she will be called on it.

"If she were a man and she was the way she is, she would get virtually no votes."

Mr Trump's remarks, while appearing to break a taboo against gender stereotypes, energised Democrats. "Keep talking, Donald Trump, keep talking," Democratic Committee National chairman Debbie Wasserman Schultz told CNN. "Every single day when Donald Trump opens his mouth, he does more to alienate women."

Mr Trump has consistently polled poorly with women. Democrats and Republicans both accuse Mr Trump of sexism over his verbal attacks on Fox News anchor Megyn Kelly and on former Republican presidential candidate Carly Fiorina, whose looks Mr Trump insulted.

On Wednesday, Mr Trump's closest Republican rival, US Senator Ted Cruz of Texas, announced that he was choosing Ms Fiorina as his running mate should he win the Republican nomination.

But US Representative Renee Ellmers of North Carolina said she believed Mr Trump could overcome his unpopularity with women voters with straight talk and a plan of action.

"This is an election unlike any other election," Rep Ellmers said. "To me, this is breaking all the rules, this is going against any of the typical history books and elections of the past."

Mr Trump's top aide and other supporters said the focus on Mrs Clinton's gender was part of Mr Trump's emerging strategy for the general election and that he had no intention of hewing to traditional rules of political campaigns.

"When he is attacked, he will respond," Mr Trump's campaign manager Corey Lewandowski told Reuters in an interview. "The campaign is going to proceed under the mantra, which we've had in this campaign from Day One, which is: Let Mr Trump be Mr Trump."

A Reuters/Ipsos poll this month showed a big gender gap for Mr Trump. About two-thirds of women have an unfavourable view of the billionaire businessman. By contrast, 54 per cent of men had a negative view of Mr Trump - a high number, but significantly lower than the negative views among women.

In taking aim at Mr Trump's comments, Mrs Clinton appeared to be relishing the fight.

"Well if fighting for women's healthcare and paid family leave and equal pay is playing the woman card, then deal me in,"she said in a victory speech in Philadelphia on Tuesday night.

Mrs Clinton has in her political career at times benefited from missteps by male candidates. Her 2000 US Senate rival, New York Republican Rick Lazio, came off as a bully when he stepped close to her on stage during a debate to demand she sign a pledge.

Early in her first presidential campaign in 2008, Mrs Clinton accused her male opponents of "piling on" and said that would prompt more women to support her.

Then Senator Barack Obama's comments during a presidential debate in New Hampshire that year that she was "likable enough" was seen as a factor in helping her win the state's primary.

If Mr Trump wins the nomination, his willingness to raise Mrs Clinton's gender and other issue could make for one of the most contentious general election campaigns in recent history.

"I don't think there's going to be any taboos with Donald Trump," said Mr James Pethokoukis, a scholar with the conservative American Enterprise Institute. "He's not going to treat her with any sort of kid gloves."