"The challenge for Donald Trump is that for the last 50 years, Americans have not voted for someone who they didn't like more than the other opponent," Luntz explained.
"That likability really does matter. And he's selling strength, he's selling determination, he's selling passion, he's selling an intensity, which is important. But Joe Biden is selling likability, and … likability does sell in American politics."
Luntz added that even if Trump's policies are more popular with voters, the president is turning people off in the way he communicates them.
"The Trump campaign is trying to prosecute Joe Biden on his policies, and there's a good reason for that," he said. "If he [Biden] talks about specifics, he's either going to alienate people in the center who he's won over or people on the left, the progressives who don't think he's progressive enough.
"In Biden's case, he wants to prosecute Donald Trump for how he communicates, for what he says … the public is more supportive of where he [Trump] stands and less supportive, quite frankly, of how he communicates it. So it makes sense," he added.
Turning to Trump's 70-minute speech accepting the GOP's renomination for president Thursday night, Luntz said the address "felt almost like a State of the Union for the first 25 or 30 minutes and then it became much more of a stump speech for the rest of it.
"The language that he used, the issues that he focused on – peace and quiet, public safety, those are the issues that Americans are increasingly caring about," he added. "And the fact that this convention is happening during Wisconsin, during Portland and Seattle, I have never seen a campaign year since 1968 that devolved more into what's happening right now.
"So the fact that we had an economic collapse, nobody is talking about it. Nobody is talking about COVID. What people are talking about now is how we restore safety and security on our streets. And that's what Donald Trump talked about yesterday."