US Republican presidential candidate Donald Trump speaks during a campaign event in Indianapolis, Indiana, United States, April 27, 2016. [Photo/Agencies]

WASHINGTON - 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 Trump and Clinton, fresh off big wins in five Northeastern state primaries on Tuesday, circled each other for a potential matchup in the Nov. 8 US presidential election, his comments portended what could be an unusually nasty campaign.

Like other controversial remarks during his White House campaign, Trump's comments drew criticism from a wide spectrum but also reinforced his image - which has been attractive to some supporters - for plain talk that defies political norms.

"The only thing she's got going is the fact that she's a woman," Trump, 69, said on Thursday on NBC's "Today" show, refusing to back down from targeting Clinton, 68, for what he called "playing the woman's card."

Trump's remarks, reaching into an area of gender attacks that is conventionally seen as off-limits, energized Democrats.

"Keep talking, Donald Trump," Democratic Committee National Chairwoman Debbie Wasserman Schultz told CNN on Wednesday. "Every single day when Donald Trump opens his mouth, he does more to alienate women."

Trump, unfazed by the criticism, told supporters in Evansville, Indiana, that he has gotten a bad rap.

"Nobody cherishes and nobody respects women more than Donald Trump," he said after being introduced by famed former Indiana University basketball coach Bobby Knight. "I will be so much better to women than Hillary Clinton."

Trump, who can be as free with his personal attacks on men, has consistently polled poorly with women. Democrats and Republicans both accuse Trump of sexism over verbal insults lobbed at Fox News anchor Megyn Kelly and former Republican presidential candidate Carly Fiorina.

On Wednesday, Trump's closest Republican rival, US Senator Ted Cruz of Texas, named Fiorina, 61, as his running mate should he win the party's nomination, a move that could help him draw women's support.

"Donald has a problem with strong women," Cruz, 45, told reporters in Fort Wayne, Indiana, before a rally. "This is not subtle, it's not complicated."

US Democratic Senator Barbara Boxer, who fended off a 2010 Senate challenge from Fiorina, said Trump is insulting all women when he attacks Clinton and other prominent women, such as Kelly, in this way.

"Either Trump has spent too much time in his Trump Tower and has no clue about what's happening, or he truly dislikes women and are quite threatened by them," Boxer said on MSNBC.