JD Vance wanted the Jeffrey Epstein files released. He repeatedly called for ending American military assistance to Ukraine. And he warned against the United States becoming involved in any overseas conflicts.

But that was all before he became President Trump’s vice president.

In his first six months in the role, Mr. Vance has emerged as a warrior for Mr. Trump, vigorously promoting and defending the president while twisting away from his past stances on a range of topics.

It’s the latest sign of how Mr. Vance, who assailed Mr. Trump in 2016 and once called himself a “Never Trump guy,” has transformed himself into a fiercely loyal ally — even when it puts him at odds with the political movement in which he is seeking to build his own following.

On Monday, speaking in front of steel plant workers in Canton, Ohio, Mr. Vance defended the administration’s decision not to release files related to the case of Mr. Epstein, a move that has provoked sharp blowback among right-wing influencers and lawmakers.

“We’re not shielding anything,” Mr. Vance said, echoing Mr. Trump’s claims that previous presidents did not do enough to shed light on the case. “Donald J. Trump, I’m telling you, he’s got nothing to hide. His administration has got nothing to hide. And that’s why he’s been an advocate for full transparency in this case.”

The stakes are high for Mr. Vance as he seeks simultaneously to demonstrate his allegiance to the president and to position himself as his natural successor. So far, Mr. Trump has repeatedly declined to endorse his vice president — or anyone else, for that matter — as worthy of inheriting his political movement.