In books like Roth's, the point of mixing history and fiction is to show that history as it unfolded was not inevitable. Events could have gone a different and darker way. Roth focuses on the year 1940, a time in which Roosevelt's bid for a third term as president in reality did run up against groups like America First, which were determined to keep the US out of World World II. Famous aviator Charles Lindbergh, in real life an Anti-Semite and America First supporter, is imagined by Roth winning the presidency. By choosing a historical figure who actually was a popular celebrity and pro-Nazi extremist and altering the facts to make him president—i.e., mixing history and fiction—Roth shows how easily and plausibly the past could have headed in a much more dangerous direction.
Roth's book is entertaining, as good fiction is, but it also functions in a cautionary way, reminding readers that it is dangerous to sit back and assume that progress is inevitable or that the "right" course will naturally be taken. It is a reminder that bad consequences can ensue, as happens to the Roth family and American Jews in the novel, if people are not alert to the reality that evil can triumph and take action to stop it.
As with the New Journalism pioneered by Tom Wolfe or Hunter Thompson's gonzo journalism, the idea is that truth can better be revealed by applying literary techniques to reality. Roth does this through his novelistic blending to show that, then and by implication now, a fascistic strand runs through American culture that wants to exclude and persecute groups perceived as "other."