President Trump Getting Bev Hills Bash Hosted by GOP Megadonor Lee Samson

If you're wondering why President Trump's going to party in super libby La La Land ... we've learned the guy who's fêting him has given nearly half a million bucks to the GOP!

Sources connected tell TMZ ... super wealthy businessman Lee Samson is the guy rolling out a very pricey red carpet for Trump this Friday in Bev Hills. He's throwing a fundraising event for POTUS and his supporters -- yes, he's got 'em in L.A., and if they're anything like Samson ... they're coming with deep pockets.

According to records, Samson -- who runs SnF Management, a hugely successful healthcare company -- donated $10k to Trump last year. Even before that, though, he was giving frequently and generously to some of Trump's former rivals ... especially Sen. Ted Cruz.

Samson's given "Lying" Ted -- as Trump used to call him -- nearly $12k since 2015 to support his Presidential run ... and his hotly contested Senate race against Beto Rourke.

He also backed Marco Rubio's Presidential campaign with a $4,360 donation. While Trump's made no secret of his hatred of late Sen. John McCain, Samson liked him enough to send $5,700 his way.

Basically, he's sprinkled a ton o' dough on multiple Republicans, but he's no dummy. He's also given to 2 Democratic Congressmen -- Ted Lieu and Brad Sherman -- we're guessing because they represent L.A. districts where Samson does business.