The party chairman said that never-Trump delegates need to recognize that voters have made their choice.
"They don’t like the idea that 14 million people ... picked someone that they didn’t want," Priebus said.
He said that opposing Trump now amounts to support for Clinton.
"It’s a binary choice," Priebus said. "It’s Donald Trump or Hillary Clinton."
A new ABC News/Washington Post poll shows Trump has all but closed the gap with Democratic rival Clinton, from her 12-point advantage in mid-June to a 4-point gap now.
Yet, just one day before the GOP convention kicks off, 60 percent of Americans say the presumptive Republican nominee is unqualified to be president.
Priebus dismissed that unfavorable poll number, saying, “Reagan had the same problem in 1979 going into 1980, and I think what it does show in reality is that [Trump] has a lot of space to grow."
He said if Trump can bring down that number of people who consider him unqualified, he will "easily" be elected president.
"He’s likable," the party chairman said. "He’s interesting to people, he’s intriguing."
Priebus also noted that Trump's choice of running mate, Indiana Governor Mike Pence, was a "mature pick" who will help the Republican ticket.
Even with Pence's help though Priebus said the convention will be a key moment in Trump's campaign.
"Thursday night’s a critical night for him, delivering a great speech, the balloon drop, the people in this country saying, 'I can see Donald Trump being in the White House. I think he’s presidential,'" he said.