The 78-year-old hooked up with the LIV Golf star sometime before he was nearly killed during a July 13 assassination attempt ... to film a video where the two would try to break a score of 50 on 18 holes from the most forward tees at one of Trump's courses.

The format for their game was simple -- they'd each hit their shots ... and whoever had the better one, they'd play from there. Normally -- in this kind of scramble -- DeChambeau's strokes would almost exclusively be used. But Trump was so locked in, the pair actually took a bunch of the former President's balls.

In fact, Trump birdied and eagled a few holes all on his own ... and he was so consistent off the tee -- he drew constant praise from his 30-year-old golfing partner.

"This is pretty impressive," DeChambeau said at one point during the round in disbelief. "I didn't expect this."

Trump knew he was on his game -- at one point, he took a shot at Biden after sticking one close to a hole.

"You think Biden can do that?" he said ... before telling Bryson, "We had an argument about golf. Can you believe it?!"

Trump later told DeChambeau during the round that he loves to golf because he plays it "well." He also told Bryson he's carded eight holes-in-one in his lifetime.

As for how the two ended up in their quest to break 50 ... they fell just short, carding a 22-under 50 right on the button.

No word yet if Biden's got plans to try his own hand with DeChambeau -- the golfer said he "reached out to both parties’ presidential campaigns" several weeks ago, but Trump was the only one down for the challenge at the time.