Candidates seeking the U.S. Republican and Democratic nominations for president have held a collective 20 major debates, but the next one could be the most unorthodox part of an already surprising campaign season.
The two parties have kept to themselves thus far, with Republicans debating Republicans and Democrats debating Democrats. That is how it always goes before each side officially names its nominee for the November general election.
The conventions for each will not be held until July, but thanks to a talk-show appearance, there could be a rhetorical showdown in the next two weeks between presumptive Republican nominee Donald Trump and Democratic candidate Bernie Sanders.
Trump appeared on Wednesday night's episode of ABC's Jimmy Kimmel Live. During the interview, Kimmel brought up the fact that Sanders' opponent, Hillary Clinton, declined to participate in a debate in California, which holds its key primary election June 7.
Kimmel then asked whether Trump would debate Sanders instead. Trump said he would, if proceeds from the presumably televised event went to charity.
On Thursday, Trump said he would agree if $10 million could be raised from the debate, possibly for women's health care issues.
"I'd love to debate Bernie," Trump said. "The only problem with debating Bernie is he's going to lose."
It did not take long for Sanders to accept the challenge.
"Game on. I look forward to debating Donald Trump in California before the June 7 primary," he said on Twitter.
Trump took part in 11 debates as the Republican field narrowed from 17 candidates to just him, the last coming in early March. For many of the events, a separate debate was held before the main one for the lower-rated Republicans because they could not all fit on the stage.
Sanders last debated Clinton in mid-April, but has seen Clinton's attention turn away from him and toward Trump as she held on to a large lead in the delegate count with fewer and fewer states left to vote. The June 7 contests will most likely clinch the Democratic nomination for her.
The official nominees from each party are not scheduled to hold their first of three debates until September 26.
Trump has often expressed sympathy for Sanders while criticizing Clinton and calling the Democratic Party's system unfair. The Democrats have hundreds of so-called superdelegates, who are party officials free to support any candidate. The vast majority are backing Clinton.
Trump had also derided the Republican process as unfair, until of course it resulted in his being the party's candidate.
Both Trump and Sanders have been surprise candidates whose position at or near the top of the presidential nomination process at this stage seemed incredibly unlikely 10 months ago, when Sanders was far behind in polls and many treated Trump's run as a joke.
If the Trump-Sanders debate does happen, it would involve an interesting dynamic between a candidate who has never been a part of the political system and one who has campaigned on a message of wanting to revolutionize the way politics in the U.S. works. It would also involve a candidate in Sanders who would have little to lose.
According to a number of national polls, if the two candidates faced each other in the November election, Trump would be the one who would lose. The most recent polls show him about even with Clinton, with Sanders still winning a hypothetical matchup by at least 10 points.