Donald Trump backs out of debate with Bernie Sanders

WASHINGTON - Days after having embraced a proposal for a televised debate against Democrat Bernie Sanders, Republican presumptive presidential nominee Donald Trump rejected the idea in a surprise move on Friday.

In familiar combative language, the outspoken real estate billionaire attributed his decision to Democratic frontrunner Hillary Clinton's virtually insurmountable lead in her party's primary race.

"Based on the fact that the Democratic nominating process is totally rigged and Crooked Hillary Clinton and (Democratic National Committee chair) Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second place finisher," he said in a statement.

"Likewise, the networks want to make a killing on these events and are not proving to be too generous to charitable causes, in this case, women's health issues," Trump added about his condition that he would take part only if the debate were to raise more than US$10 million (S$13 million) for charity.

"Therefore, as much as I want to debate Bernie Sanders - and it would be an easy payday - I will wait to debate the first place finisher in the Democratic Party, probably Crooked Hillary Clinton, or whoever it may be," he said.

Trump and Sanders prompted widespread anticipation by agreeing to the unusual idea for a debate earlier this week, with their campaigns saying it should be held in a stadium amid speculation the event would generate record ratings.

The Sanders campaign said in a statement shortly before Trump's announcement that it had received offers from at least two television networks.

Trump won enough delegates on Thursday to clinch the Republican Party's nomination to run in November's general election.

Clinton - who dismissed the idea of a Trump-Sanders debate as a "joke" - on Monday reneged on her own campaign's previous agreement to debate the Vermont senator this month ahead of a potentially decisive primary in California and several other states on June 7.

Despite her all-but-certain Democratic primary victory, polls show her lead over Sanders in California has evaporated.