Privately, some of Mr. Trump’s 2016 aides have said they are pessimistic about his path to 270 electoral votes after his party’s midterm defeats in states like Michigan, Pennsylvania and Wisconsin. An Associated Press poll on Wednesday showed that Mr. Trump’s overall approval rating had fallen to 34 percent, with his support among Republicans dipping below 80 percent — a startling turn for a president who strives for total control of the G.O.P., and has usually achieved it…

Mr. Trump is especially fixated on two well-known Democrats, speaking frequently about Joseph R. Biden Jr., the former vice president whom Mr. Trump regards as his most dangerous potential opponent, and Senator Elizabeth Warren of Massachusetts. Some of his advisers are more preoccupied with two other would-be challengers, who would offer a starker generational contrast with the 72-year-old president: Senator Kamala Harris of California and Beto O’Rourke, the former Texas Senate candidate.

In this volatile moment, however, Mr. Trump and his aides have been just as focused on heading off competition from fellow Republicans. In addition to helping direct a resolution of support from the R.N.C. — Mr. Trump tweeted approvingly Saturday morning, suggesting re-election “should be easy’’ — Mr. Stepien, the former White House political director, has been leading an effort to ensure there is no insurrection at next year’s convention in Charlotte, N.C.