Republican Donald Trump and Democrat Bernie Sanders on Thursday explored staging an unconventional U.S. presidential debate that would sideline Democratic front-runner Hillary Clinton and create a television spectacle that could attract huge ratings.

The two men — a billionaire and a democratic socialist — expressed interest in a one-on-one encounter in California even though Republican and Democratic presidential candidates traditionally do not debate each other until the parties have selected their nominees.

"I'd love to debate Bernie," Trump told reporters in North Dakota, after he secured enough delegates to clinch the Republican presidential nomination. "I think it would get very high ratings. It would be in a big arena."

Basking in his newly sealed nomination at a later campaign rally in Billings, Mont., Trump said he expected to put 15 states in play in the general election, compared with three or four for a traditional Republican. He named California, Washington and Michigan among others.

How are Trump and Sanders supporters gonna sit in the same room together without a fight? <a href="https://twitter.com/hashtag/BernieTrumpDebate?src=hash">#BernieTrumpDebate</a> <a href="https://t.co/AU7trWDEUQ">https://t.co/AU7trWDEUQ</a> —@lucas_tap23

Trump spokeswoman Hope Hicks said in an email there were no formal plans yet for a debate. But Sanders campaign manager Jeff Weaver told CNN there had been "a few discussions" between the campaigns about the details.

"We hope that he will not chicken out," Weaver said. "We hope Donald Trump has the courage to get on stage now that he said he would."

Sanders, a U.S. senator from Vermont, is running far behind Clinton in the race for the Democratic nomination for the Nov. 8 presidential election. But a nationally televised debate with the presumptive Republican nominee would be a big boost to his chances in the California primary on June 7, when Clinton is likely to clinch the nomination.

'They'll have to pay a lot of money for it'

Trump said a debate with Sanders could raise up to $15 million for charity.

"I'd love to debate Bernie, but they'll have to pay a lot of money for it," he said. The idea was hatched during an appearance by Trump on ABC's Jimmy Kimmel Live late on Wednesday. Kimmel said he asked Trump about the debate at the suggestion of Sanders, who is scheduled to appear on the show on Thursday night.

Appearing a night later on the same talkshow, Sanders said of Clinton's refusal to debate in California: "I think it's kind of insulting to the people of the largest state in the United States of America."

He said he is genuinely interested in a faceoff with Trump, saying the pair sees the world in vastly different ways, adding that "the goal would be to have it in some big stadium."

I am delighted that <a href="https://twitter.com/realDonaldTrump">@realDonaldTrump</a> has agreed to debate. Let’s do it in the biggest stadium possible. —@BernieSanders

Clinton, who backed out of an agreement to debate Sanders before the California vote, said she did not think a Trump-Sanders showdown would happen.

"This doesn't sound like a serious discussion. I'm looking forward to debating Donald Trump in the general election. I really can't wait to get on the stage with him," she told CNN in a phone interview.

Fox News shows interest

A Fox News spokeswoman confirmed the network was trying to host a forum with Trump and Sanders. Representatives from other networks did not immediately respond to requests for comment.

"If it does come to pass, it would generate enormous ratings," said Alan Schroeder, a Northeastern University professor who has written extensively about presidential debates. "They are from two different planets. You have a real personality contrast. It would dominate media coverage."

Can't wait for the... <a href="https://twitter.com/hashtag/BernieTrumpDebate?src=hash">#BernieTrumpDebate</a> <a href="https://t.co/Dp8MXATbK8">pic.twitter.com/Dp8MXATbK8</a> —@robbiebliny

Sanders, who has promised to continue his campaign through the Democratic nominating convention in July, has said he will do everything he can to ensure that Trump does not win the White House.

"Smart and bold move by Sanders," Democratic strategist Brad Bannon said. "The Clinton people are furious but Bernie wins points for being so aggressive."

Clinton has tried to woo Sanders supporters as she turns her attention to the general election. But some Democrats worry his supporters — who are largely young, working-class and disillusioned with the Democratic Party establishment — will turn instead to political neophyte Trump, who has championed a populist agenda.

'It crosses a line'

The debate would give Trump a national forum to criticize Clinton and try to win over Sanders supporters ahead of an expected Trump-Clinton general election contest, Democratic strategist Chris Kofinis said.

"I think Sanders should think long and hard about giving Trump a forum," Kofinis said. "It crosses a line, but apparently in this election there is no line."

Dale Ranney, 62, a Trump volunteer who has been to 21 of his rallies, said she would be delighted to see Trump and Sanders debate.

"I think it's a great idea, any time you can get more information to the people, absolutely," Ranney said. "Having Trump debate a socialist? Absolutely. Go for it."