JUSTIN BIEBER SORRY, GOP ... I'M NOT FOR SALE

Justin Bieber got a $5 million offer to perform at a Republican event during the GOP Convention, but turned it down after his manager considered quitting andLeBron James urged him to reject it ... sources tell TMZ.

Sources connected with CAA -- the talent agency which reps Bieber and received the offer -- tell us a promoter contacted Justin's people with the offer, saying he would perform for 45 minutes at a venue near Quicken Loans Arena, where the RNC was held.

We're told the promoter assured Justin's people the event was "not political" and Justin would not be endorsing Trump or any GOP policies.

According to the contract, the $5 million was bankrolled by GOP donors. The money was to be paid "up front" -- before Justin even arrived. The donors would also cover production expenses.

It represented the biggest single payday ever for Justin. Or, for that matter, just about any other singer, and he seriously considered doing it ... partly because he's Canadian and didn't have a dog in the fight and partly because of the money.

Manager Scooter Braun, a big Hillary Clinton supporter who even held a fundraiser for her at his L.A. home, had big problems with the offer ... as did CAA. Scooter and CAA felt, regardless of the representations, this was "100 percent political."

We're told Scooter wasn't heavy-handed at first, and talked to Justin about putting Black Lives Matter banners up in the venue. We're told Braun was told he could not display BLM banners, but the donors would accept banners that read "All Lives Matter." The donors also made it clear ... Justin could say nothing disparaging about Trump or the GOP.

We're told Scooter told Justin it was up to him if he wanted to do the show, but he said he couldn't continue to represent the singer if he did, out of principle. Scooter felt the GOP was using Bieber as its tool.

Our sources say the promoter told Scooter ... LeBron James himself was also going to attend an event in Cleveland, welcoming the GOP without taking a political stand. The sources tell us Justin's people got in touch with LeBron's people, and they were told LeBron would actually not be in Cleveland for the convention and urged Justin not to go as well.

On top of all that, we're told Justin's band, comprised entirely of African-Americans, did not want to do the show.

In the end, Justin turned the offer down.

TMZ