Bernie Sanders' openness to debate Donald Trump ahead of California's primary -- an idea first floated during a "Jimmy Kimmel Live" interview with the presumptive GOP nominee Wednesday -- has some Democratic lawmakers fuming.

Sen. Joe Manchin, D-West Virginia, told Politico Thursday that the possibility of a debate was "bulls***."

"That confirms what we've been saying," Manchin told the news site. "Why would you expect Bernie should be considerate or be nice or be working to bring everyone together? Why? He's not a Democrat."

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California Sen. Dianne Feinstein, whose state holds its primary on June 7, posited that the debate was a publicity ploy.

"What he's trying to do," Feinstein told Politico, "is bring attention to himself."

Michigan Sen. Gary Peters also told the news outlet, "I don't know why he would do that."

"I think it's time to start to winding down the primary," Peters said. "It's time to move on."

The naysaying members of the Senate's Democratic caucus underscore the increasing tensions between the party and Sanders, whose sometimes vicious primary battle against front-runner Hillary Clinton has led to some division in the ranks.

Other Democrats, however, have said they believe the debate would have a positive outcome, including Clinton surrogate Claire McCaskill, a Missouri senator.

"The more Donald Trump gets exposed in the context of public policy, the better it is for our country," McCaskill told Politico.

Oregon Sen. Jeff Merkley, Sanders' single supporter in the Senate, agreed.

"Here is Trump, who is posing as a defender of working Americans and he hasn't done one thing for working Americans his entire lifetime," Merkley said, according to Politico.

Whether the debate will actually happen before the California primary still remains to be seen.

Trump's own campaign seems undecided about the event, with staffers telling CBS News early Thursday that their candidate had just been joking when he agreed to it.

But in a North Dakota press conference later that day, Trump said he would "love to debate" Sanders -- provided some conditions were met.

"Maybe if we can raise for women's health issues or something-- if we can raise 10 or $15 million for charity....I think it would get very high ratings," Trump told reporters Thursday. "It should be in a big arena somewhere, and we can have a lot of fun with it."

While some television networks were in talks with Sanders' campaign manager Jeff Weaver about hosting the debate, only one media outlet so far has offered to pony up some cash for the event.

Cenk Uygur, who hosts the online left-leaning video show "The Young Turks," announced Thursday that he would give $1 million to charity to stage the debate.

"It's a very real offer -- the charity could be decided upon by Donald Trump and Bernie Sanders," Uygur said in an announcement video.