"Had he understood politics a little bit more, had he started a little bit earlier and done things a little bit differently, he would be the United States senator from Georgia now," said Atlanta-based Republican strategist Tom Perdue, who supported Cain in 2004 but didn't work on his campaign.
Perdue believes the 2004 race was a vital political education for Cain.
The former Godfather's Pizza CEO went into that race "naive about politics," thinking he could command the political arena like he did the world of business. He came out of it with a better understanding, Perdue said.
"He learned from his mistakes," Perdue said. "That doesn't mean he's going to be president of the United States. But he learned, and that in itself tells you that he's a smart man."
Though Cain is commonly depicted as a political novice who's more or less lucked into his current position topping several national polls, he's not the newcomer he once was, Perdue said. Cain knows now, as he didn't then, that personal charisma and attractive ideas aren't enough -- you also have to be able to organize at the grassroots level, and you have to raise money.
The Senate race pitted Isakson, at the time a member of the House of Representatives, against another sitting member of Congress, Mac Collins, who also sought to outflank Isakson on the right.
Cain got in the race later than those two. As he does now, he styled himself as the rock-the-boat outsider against the career politicians. As he does now, he trumpeted a bold, regressive and probably unfeasible tax plan -- back then, it was the "fair tax" national sales-tax proposal.
"One thing you could say about him even then is he was bold. He was thinking outside the box," said Dave McCleary, who got to know Cain as vice chair of the Fulton County Republican Party at the time. He is now the Georgia state director of Cain's presidential campaign.
McCleary recalled seeing Cain speak at GOP events in 2004. "I was impressed not only with his speaking ability, but with his sincerity," he said. "Politicians tend to be a little shallow, but Herman Cain was extremely sincere. It didn't matter whether you were the waiter or the CEO, he spoke to everybody the same."
Both Cain and Collins saw their best chance in attacking Isakson for not opposing abortion strongly enough.
Though Isakson said he believed in just three cases where abortions should be allowed -- rape, incest and to save the life of the mother -- Cain and Collins believed in only one exception, for the life of the mother.
Many in the state's active Christian right were willing to give Isakson the benefit of the doubt until, in the middle of the campaign, Isakson voted in Congress to allow members of the military to get abortions at overseas military hospitals. That sent social conservatives into an uproar. Georgia Right to Life jointly endorsed Cain and Collins.