Republican Donald Trump and second-placed Democrat Bernie Sanders could be heading to an unconventional US presidential debate in California, in the latest twist to an unusual campaign.The two candidates expressed an interest in squaring off in a one-on-one encounter that would leave Democratic front-runner Hillary Clinton on the sidelines.Sanders, a US senator from Vermont, is running behind Clinton in the race for the Democratic presidential nomination, while Trump on Thursday secured the delegates needed to clinch the Republican nomination. Republican and Democratic presidential candidates traditionally do not debate each other until the parties have selected their respective nominees.In an appearance on ABC's "Jimmy Kimmel Live" late on Wednesday, billionaire Trump said he was willing to participate in a debate with Sanders, a democratic socialist."If I debated him, we would have such high ratings," the former reality TV star said. "I think I should ... take that money and give it to some worthy charity."Sanders appeared to agree in a post on Twitter on Thursday."Game on," he tweeted. "I look forward to debating Donald Trump in California before the June 7 primary."The hashtag #BernieTrumpDebate began trending in the United States with news of the possible debate.Trump spokeswoman Hope Hicks said in an email on Thursday that there were no formal plans yet for such an event. But Sanders spokesman Michael Briggs said the campaign plans to reach out to the Trump camp to discuss the idea."We look forward to following up and making sure it happens," Briggs said.Kimmel said he asked Trump about the debate at the suggestion of Sanders, who is scheduled to appear on the show Thursday night.