JD Vance, a 40-year-old senator who transformed himself from a biting critic of Donald J. Trump to one of his fiercest defenders, was elected the next vice president of the United States on Wednesday, becoming the third youngest and among the least experienced and most polarizing politicians ever to hold the office.

The country’s 50th vice president will be sworn in just two years after assuming his first public office as a senator from Ohio. Mr. Vance is unlike any other vice president before him in the modern era: None has started the job with such an extensive public record of condemning his or her boss.

He rose to national prominence with his 2016 memoir, “Hillbilly Elegy,” a best seller that liberal voters devoured to better understand Mr. Trump’s victory and the frustrations of the white working class that had put him in the White House. Mr. Vance also seemed an ideal translator for Blue America as a Midwestern conservative who detested the new president, likening him in an essay to “cultural heroin.”

But Mr. Vance then started professing a change of heart and mind about the leader of his party as he prepared his own initial run for office. Mr. Trump not only forgave his young convert but also rewarded him with a game-changing endorsement in a fiercely competitive, four-way Senate primary and then, in the general election, helped push Mr. Vance’s underperforming campaign over the finish line.

Now, Mr. Vance is more politically indebted to Mr. Trump than any other vice president has been to the top of the ticket in modern times, said Joel Goldstein, a professor emeritus at the St. Louis University School of Law who has spent decades studying the vice presidency.

“When you have someone whose entire political career is owed to Donald Trump, it really raises a question as to whether JD Vance is somebody who is capable of telling him when he’s wrong,” Mr. Goldstein said.