How might Herman Cain hold on to his spot atop the polls?

It’s all about likability, as Cain travels a path once trod by none other than Ronald Reagan, our powerhouse guests Sam Donaldson and Frank Luntz agreed on ABC’s “Top Line” today.

“There’s something diff about Cain and he was much stronger than we realized at the time,” Luntz, a Republican pollster and strategist, told us. “Which is that likability still matters in American politics.”

“Absolutely. I give you Ronald Reagan,” added Donaldson, who covered Reagan extensively as ABC News’ chief White House correspondent.

“Don’t forget that Ronald Reagan made many, many mistakes about the specifics and people who sat in the chairs that you two gentlemen sit roasted him over it,” Luntz continued. “And in the end the American people decided the specifics were less important than the principles that they represented.”

Jonathan Karl and I had a spirited and engaging conversation with Donaldson and Luntz today, on faith in politics – Luntz is moderating a faith forum with several of the candidates this weekend in Iowa – Romney’s struggles to close out his opponents, plus Gingrich’s rise in the polls.

“I predict that on Jan. 3 [the date of the Iowa caucuses], Newt will be over 20 percent in the national polls,” said Luntz, who advised Gingrich with his “Contract with America” in 1994.

Said Donaldson: “Newt’s got the kind of baggage that if he wins, it will really bring us forward from the standpoint of what matters… to the things some Republican voters really care about.”