“I can’t imagine that he’s not,” Mr. McConnell said when asked whether Mr. Trump was comfortable with Mr. Romney, who on Friday made official his long-expected candidacy for a Senate seat from Utah. “We don’t want to lose the seat, and this looks like a pretty formidable candidate.”

Mr. McConnell, the majority leader, said that Mr. Romney would enter the Senate with a stature similar to what Hillary Clinton brought as a junior senator. “The best way to think about that, and I told him this a couple of months ago, I said: ‘You’ll be a freshman like Hillary Clinton was,’” Mr. McConnell recalled. “He will come in here with a level of national identity and respect that will make him effective from Day 1.”