Few political reversals have been as dramatic and complete as the transformation of Senator JD Vance of Ohio from a self-proclaimed Never Trumper into former President Donald J. Trump’s new running mate.

Democrats have used Mr. Vance’s turnabout to paint him as untrustworthy, while Republicans have described his shift as another sign of Mr. Trump’s influence on the party. Whatever the case, Mr. Vance’s U-turn was a gradual one that played out in stages during the past eight years.

In many ways, Mr. Vance’s life has echoed the theme of change. He twice changed his name, a result of a tumultuous upbringing, and was baptized into the Catholic Church just five years ago. He has been unapologetic about his political change of heart, even weaponizing his flip-flop by blaming the news media for distorting voters’ views of Mr. Trump.

“I was critical of President Trump,” Mr. Vance said this week in Ohio. “But he cares about human beings. He is not the caricature or the lie that the media has told you that he is.”

But a review of Mr. Vance’s writings and interviews since 2016, when he first rose to fame as the author of “Hillbilly Elegy,” reveals a more complicated process behind his political conversion than the “media lies” explanation he often uses.

A fierce Trump critic during the promotion of his memoir, Mr. Vance softened during Mr. Trump’s first two years in the White House. He actively began defending Mr. Trump during the second half of the president’s first term, before effectively apologizing for his own rebukes during his own Senate run. By the time he was mentioned as a potential running mate, Mr. Vance ranked as one of the most combative foot soldiers in the former president’s Make America Great Again movement.