Senator J.D. Vance of Ohio, the newly announced running mate to former President Donald J. Trump, has gone on a rapid journey over the past eight years from best-selling author and outspoken Trump critic to one of Mr. Trump’s staunchest defenders and, now, his would-be second in command.

Before running for office, Mr. Vance, 39, was known as the author of “Hillbilly Elegy,” a best-selling memoir recounting his upbringing in a poor family that also served as a sort of sociological examination of white working-class Americans. The book was published the summer before Mr. Trump’s election in 2016, and many readers looked to it after his victory as a sort of guide to understanding Mr. Trump’s support among white working-class communities.

Mr. Vance himself harshly denounced Mr. Trump during his 2016 campaign. But by 2022, he had embraced Mr. Trump, winning a crowded Republican Senate primary with his backing and becoming a reliable pro-Trump voice in Congress.

Here is more on Mr. Vance’s background and views.