For some time now, Tesla and SpaceX maestro Elon Musk has been on the President’s curiously named Strategic and Policy Forum – essentially an advisory council for some of America’s top business leaders and innovators. Apart from investment into space exploration, there’s actually little Musk and Trump agree on.

The entrepreneur has defended his presence on the council, presenting himself as a potentially moderating influence on the more extreme positions that Trump has taken, including the executive orders on immigration bans and the opposition to the Paris climate agreement.

Musk has apparently reached a breaking point, however – he’s announced that he will step down from the council if Trump does indeed withdraw from the groundbreaking climate accord.

Just yesterday, as it was becoming apparent that Trump had made his decision to withdraw from the Paris agreement, Musk took to Twitter to express his concerns. “Don’t know which way Paris will go,” he said, “but I’ve done all I can to advise directly to POTUS, through overs in WH & via councils, that we remain.”

When someone asked him what he will do if Trump does take America out of the pact, Musk replied: “Will have no choice but to depart council in that case.”

People will probably wonder why Musk has stayed on for so long, as it appears little of his advice has been taken on by the President. Perhaps this agreement is just the straw that broke the camel’s back.

Musk’s appointment on the council has long been considered by fellow Silicon Valley companies and entrepreneurs as somewhat unusual – most of them, with a few bizarre exceptions – are definitely not in favor of the Trump administrations and its behavior in general.

As reported by Politico, some of the big hitters, including Apple, Facebook, and Google are re-running a full-page pro-Paris advertisement today in The New York Times and The Wall Street Journal. It’s unlikely that Trump will pay it any attention, but it’ll serve as a reminder that American tech businesses, among many others, are holding out hope that the US will remain in the pact.

Musk won’t be the first big name to step down from the advisory role, by the way. Back in February, Uber CEO Travis Kalanick stepped down after a public backlash forced his hand.

Either way, it’s a growing sign that the President cannot be negotiated with for the most part. Even his own daughter Ivanka, who holds a lot of sway, failed to convince him to remain in the Paris agreement.