As you know, Clooney fired the first shot with his NYT op-ed Wednesday, saying Biden can't win the election and called for him to drop out of the race. In the letter, Clooney referenced last month's record-breaking fundraiser, saying ... "The Joe Biden I was with three weeks ago at the fund-raiser was not the Joe 'big F-ing deal' Biden of 2010. He was the same man we all witnessed at the debate."

Now ... a source connected to the Biden campaign, who is deeply familiar with the planning that went into the Hollywood fundraiser, is dismissing Clooney's critique and pointing the finger squarely at him.

We're told the event took months to plan, and during that process, Clooney left the Biden campaign with only one possible date it could be held ... June 15. The Biden campaign made it abundantly clear that date was not ideal, because Biden would be attending the G7 Summit in Italy on the 14th, but we're told Clooney wouldn't budge.

Of course, team Biden didn't want to miss out on the opportunity to have a fundraiser headlined by Clooney, so they agreed. As a result, Biden had to make a 13-hour flight from Italy to L.A. on the same day as the event.

Financially, the event ended up being a smash hit ... TMZ broke the story, the campaign raised $28M heading into the fundraiser making it the single largest Democratic fundraiser in U.S. history.

As for Clooney's claim Biden wasn't the same person he was used to seeing ... we're told the President's grueling travel -- to meet Clooney's demand -- might have something to do with that.

We reached out to the Biden campaign, who pointed us to the letter the president sent to Democrats in Congress earlier this week ... saying he has no intention of exiting the election.