Donald Trump's dramatic ascension to the presumptive Republican presidential nomination is "perilous to the republic" and threatens to lead the U.S. into fascism, says Robert Kagan, a senior fellow at the Brookings Institution."This is how fascism comes to America," Kagan, a noted historian, writes in the Washington Post . "Not with jackboots and salutes … but with a television huckster, a phony billionaire, a textbook egomaniac 'tapping into' popular resentments and insecurities."Trump's rise is being aided by "an entire national political party — out of ambition or blind party loyalty, or simply out of fear — falling into line behind him," Kagan adds."Fascist movements, too, had no coherent ideology, no clear set of prescriptions for what ailed society … Successful fascism was not about policies but about the strongman, the leader (Il Duce, Der Führer), in whom could be entrusted the fate of the nation."Whatever the problem, he could fix it. Whatever the threat, internal or external, he could vanquish it, and it was unnecessary for him to explain how."Kagan says that while less than 5 percent of eligible voters have voted for Trump, the billionaire businessman will control the powers of the Justice Department, the FBI, the intelligence services and the military if he wins the general election."Who would dare to oppose him then? Certainly not a Republican Party that lay down before him even when he was comparatively weak," he writes.Kagan is the author of " The World America Made ," published by Vintage.