USA and Australia are about to duke it out in the World Cup ... and ya know who's shaking in his Chelsea boots?! TMZ DC's Charlie Cotton -- 'cause he's making a wager with Senator Tim Kaine!!

Cotton -- who is Australian A.F. -- ran into Hillary Clinton's former running mate on Capitol Hill Thursday afternoon ... and mentioned how they're enemies as soon as the two countries kick-off in Seattle.

Kaine -- clearly rooting for the home team -- suggested the two take part in a beer bet (Foster's and Samuel Adams, obviously) ... but Cotton threw a ton of alternatives in his direction, and he shut down each and every single one.

Our guy really thought he would be able to get Kaine to agree to a "shoey" if the Aussies won ... and for those who don't know, that's the popular Australian drinking act when someone chugs a beer out of a sneaker. No chance.

He also suggested they exchange clothes for a day ... and no disrespect to C.C., but we understand why Kaine wasn't down with that idea either.

Kaine said he's no newbie to lighthearted agreements ... and pointed out how he scored some bragging rights and BBQ courtesy of Sen. Ted Cruz after his Nationals beat the Astros in the 2019 World Series.

While they haven't reached common ground just yet, there are still a few hours to finalize the deets.

For all you footy fans, every FIFA World Cup™ matchup is available on the FOX One app.