Ron Paul did not call Huckabee a Fascist, but he did point out that everything is not always what it seems. That was his point. Frankly, I do not think Huckabee supports a Mussolini style Corporate/Totalitarian State. But his "Christianity" seems a bit confused, when it is used to justify his past support for Hispanic migrants taking the jobs of native Arkansans. Neither the concept of a Universal God, nor the evangelical pursuit of winning adherence to that Universal God, can rationally support the principle of failing in a duty to provide for your own posterity. It is in that failure that Governor Huckabee's wearing of Faith upon sleeve becomes really suspect.
But all Dr. Paul was really saying by the Sinclair Lewis quote, is don't trust appearances--or the self-serving representations of those seeking political office or power.
While American conservatism in its truest sense is closest to classical liberalism and free market economics as espoused by John Locke, Adam Smith, and Thomas Jefferson, there is also in American history an authoritarian, pro-big government, nationalist tendency, separate from the leftist tendency. The 1920s Ku Klux Klan was one manifestation, but so were the more authoritarian Federalists like Alexander Hamilton at the beginning of the Republic, the Know Nothings of the 1850s, the Southern Populists around the turn of the last century, the various fascist groups of the 1930s, and the "law 'n' order" movement first represented in the George Wallace third party Presidential candidacy 40 years ago.
Unlike Marxism, fascism is not a thoroughgoing worldview, but nationalist, populist, and authoritarian sentiments combined with nostalgia for some bygone era. Large corporate interests have aligned themselves with these movements, much as modern American corporate entities support liberal Democrats and RINOs, in a cynical play to back the winner. Remember that the Japanese zaibatzus, or business trusts, supported their country's militarism, as did the Krupp and Thyssen interests did for Hitler in Nazi Germany. In its most extreme forms, like Nazism, it differs little from Communism in terms of totalitarian state power and the suppression of freedom. American authoritarians have never had a person like the fictional Nehemiah Scudder of sci-fi author Robert Heinlein's Future History series, and our society has long been too diverse to unite the nation against a hated minority.
I would argue that the main differences between the Klan and the Blackshirts were not in ideology, but in organizational leadership and the different natures of America and Italy. Well over 95% of Italy's citizens are native Italians with a common culture and language, despite regional differences. Catholics, Jews, blacks, and other religious and racial minorities like Mormons and American Indians were probably over one-third of the U.S. population in the 1920s. A Klan coup would have led to a second civil war, probably as nasty as the one Spain experienced in the 1930s.
For all his flaws, I do not see Mike Huckabee as a real life Nehemiah Scudder or an American Benito Mussolini. Perhaps Ron Paul does not either. However, it is reasonable to draw the implication that he thinks so from his citation of the Sinclair Lewis line.