WASHINGTON — Senator Mitch McConnell said in an interview on Friday afternoon that President Trump should rally behind the Senate candidacy of Mitt Romney despite his blistering criticism in 2016, arguing that Mr. Romney’s potent bid was an illustration of the Republican Party’s improving fortunes entering a challenging midterm campaign.

“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.”

His glowing appraisal stood in contrast to that of the White House, which aggressively lobbied Senator Orrin G. Hatch, the veteran lawmaker who currently holds the Utah seat, to seek an eighth term in large part to block Mr. Romney’s path. Some close to Mr. Trump worry about giving Mr. Romney a platform from which he could run for president again, and Mr. Romney said in remarks Friday night to Utah Republicans that he would not shy away from taking on the president if necessary.