Donald Trump lashed out at his fellow Republicans in a speech to a conservative crowd in Nashville, Tenn., on Saturday.

The bombastic billionaire said that inside-the-Beltway GOPers “become weak” ­after they’re elected.

“They walk into these magnificent buildings . . . and they go ‘Oooh, I made it,’ ” he mocked. “ ‘I’ll vote for ObamaCare extensions, I’ll do whatever you want.’

“Well, it won’t happen to me, I promise you,” he said.

Trump told the crowd at the right-wing National Federation of Republican Assemblies, many of whom are aligned with the Tea Party movement, “You have not been treated fairly.

“The Tea Party people . . . get beat up all the time by the media,” he said

Later, Trump told reporters that he will announce whether he’ll mount a fallback third-party run at the presidency “very soon.”

“I think a lot of people are going to be very happy,” he said.