Republican nominee Donald Trump has backed out of an offer to debate Democratic candidate Bernie Sanders.
Mr Trump’s campaign said it would be “inappropriate” to debate the second place finisher since Hillary Clinton is poised to win the nomination.
Mr Sanders’ campaign had said it was willing to participate in a debate with the billionaire businessman, who just cinched the Republican nomination.
The debates would have included contributions to charity.
The Sanders campaign said it would “look forward to a substantive debate that will contrast the very different visions that Senator Sanders and Mr Trump have for the future of our country.”
Mr Trump had joked that he was willing to debate Mr Sanders during an appearance on the Jimmy Kimmel programme.
Analysis: Anthony Zurcher, BBC North America reporter
If the drama over the on-again, off-again Trump-Sanders debate was the first skirmish of the Clinton-Trump general election battle, you can mark Mrs Clinton down as the winner.
While Bernie Sanders and Donald Trump danced around the prospect of an unprecedented 1 v 2 pre-convention showdown, the former secretary of state didn’t take the bait and held her tongue.
Instead Mr Sanders comes off as a bit desperate, looking for any opportunity to take a moment in the spotlight, even if it would benefit the Republicans and undermine Democratic unity efforts.
And Mr Trump was put in the unusual position of having to claim the event – which would have been compelling television that attracted an audience of tens of millions – was “inappropriate”.
Given that last month a near-hopeless Republican candidate, Ted Cruz, announced his vice-presidential running mate just days before dropping out, it would have been fitting if the primary season had concluded with the nearly vanquished Mr Sanders in a general-election style debate with Mr Trump.
But it was not to be. Some ideas are just too far-fetched, even for this campaign.
“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,” the Trump campaign said in a statement.
Mr Sanders told reporters on the campaign trail on Friday that he hopes Mr Trump changes his mind.
“Well Mr Trump, what are you afraid of?” he said, calling him a “bully”.
Mr Trump called the Democratic nominating process “rigged” and that Mrs Clinton and Democratic National Committee Chairwoman Deborah Wasserman Schultz would not allow Mr Sanders to win the nomination.
He has taken to calling Mr Sanders “Crazy Bernie” in the past few weeks.