Mr. Carson, who grew up poor in Detroit and rose to success as a doctor at Johns Hopkins University Hospital in Maryland, made the paperwork change in his new home state of Florida. He filed the paperwork in Palm Beach County.

“It’s truly a pragmatic move because I have to run in one party or another. If you run as an independent, you only risk splitting the electorate,” Mr. Carson told The Washington Times in an interview Tuesday night shortly after making the change. “I clearly would not be welcome in the Democratic Party, and so that only leaves one party.

“Fortunately, the fit is pretty good,” he added. “I believe in reasonable sized government. I believe in personal responsibility. I believe in individual freedom. And I believe in creating an environment to let free markets grow our economy. And all of that is very consistent with being a Republican.”