Republican Donald Trump and second-placed Democrat Bernie Sanders could be heading to an unconventional U.S. 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 U.S. 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.