WASHINGTON (AFP) - Donald Trump hit out hard at Hillary Clinton and Ted Cruz on Sunday, sounding unapologetic two days ahead of a key primary in Indiana he says will decide the Republican presidential race.

Speaking on Fox News Sunday, the Republican frontrunner concentrated on attacking Clinton, doubling down on his much-criticized statement that the likely Democratic nominee's only appeal to voters is the "woman's card".

"She's done a lousy job in so many ways and even women don't like her," he said. "But it is the woman's card and she plays it and I will let you know in about six months whether or not she plays it well, but I don't think she'll play it well."

"If she were not a woman she wouldn't even be in this race," he added.

The real-estate billionaire also targeted Cruz, dismissing his closest Republican rival's criticism of Trump's endorsement by former boxing champion Mike Tyson, whom Cruz called a rapist.

"No big deal, I didn't have a meeting or anything, I haven't seen Mike in years," Trump said.

"This guy is a real liar," he added of Cruz. "That's why we call him lyin' Ted Cruz."

Trump dismissed Cruz's endorsement by Indiana Governor Mike Pence this week, calling it "the weakest endorsement anyone has seen in a long time".

"I think he gave me more of an endorsement than he gave Cruz," he added.

Asked whether a victory for Trump in Indiana on Tuesday would clinch the nomination for him, Trump said, "Yes, it's over," adding, "I think it's over now."

A Wall Street Journal/NBC News/Marist poll released on Sunday gave Trump a 15 point lead in Indiana.