The renewed Hillary Clinton FBI email mess could raise "constitutional issues" if the Democratic nominee were to win the White House, said Larry Kudlow, a CNBC senior contributor and informal advisor to the Donald Trump campaign.

But in the same CNBC interview on Wednesday, Keith Boykin, a former Bill Clinton White House aide, countered that actual pending lawsuits against Trump cast a shadow over the GOP nominee, and the FBI Clinton investigation may turn out to be nothing.

Kudlow got the discussion rolling by saying: "The possibility of a redo on the whole [FBI email] investigation makes [Clinton] the unsafe candidate."

If Clinton were charged, he asked: "Does she pardon herself?"

"This is a criminal investigation," Kudlow said, referring to the FBI's review of emails possibly related to Clinton's private server that were found on Anthony Weiner's laptop during a sexting investigation of the ex-congressman. Weiner is the estranged husband of top Clinton aide Huma Abedin.

"She could win. She could be inaugurated. And the investigation is still going to go on," Kudlow added, claiming a hampered Clinton would not be able to work with Republicans on the issues.

Sitting next to Kudlow on "Squawk on the Street," Boykin dismissed that argument.

"In 2000, we had a constitutional crisis, potentially, because [George W. Bush] won the electoral vote but lost the popular vote," he said. But Bush "still was able to come in and run the country will some sort of mandate. That's not going to be an issue for Hillary Clinton."

Putting the Clinton email issue aside, Kudlow said Trump has been doing better in recent days, and the tightening polls show it.

"[Trump] has been on message for six, seven, eight days, which is the 'eighth wonder of the world,'" Kudlow joked. "He's actually stayed on message" by talking about "draining the swamp in Washington" and getting rid of Obamacare.

But Boykin pointed out, as do many experts, the map of potential electoral college votes still favors Clinton. In addition to Clinton locking down the blue states, he said: "I think she's [also] doing well in states where she was not supposed to do well."