Julian Zelizer is a history and public affairs professor at Princeton University and co-host of the "Politics & Polls" podcast. In January, Norton will publish his new book with Kevin Kruse, "Fault Lines: A History of the United States Since 1974." Follow him on Twitter: @julianzelizer . The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) Although Roger Ailes, the former CEO of Fox News, died in May 2017, he continues to shape our political world through his TV network. And, in the months since, President Donald Trump has consummated the marriage between the network and the Republican Party.

Fox News, which Trump and his surrogates appear on regularly, has been largely supportive of the President's agenda. This is not a surprise since what we consider to be Trumpian politics is in the DNA of the network. A new documentary, "Divide and Conquer" (opening December 7) makes this clear through a disturbing look into Ailes, the entrepreneur who built Fox News.

The director, Alexis Bloom, told me she wanted to make this film because we "live in Roger's world" and need to know more about the "scaffolding of America" from which Trump came.

When Rupert Murdoch hired Ailes to head his network in 1996, the Republican political operative dreamed of creating a partisan news-entertainment network that sold conservatism in an angry and eye-popping way. The station would appeal to Americans who lived in "fly-over country" and felt their voices were being ignored by traditional media outlets.

When the channel's viewership boomed as a result of its coverage of President Bill Clinton and Monica Lewinsky, Ailes doubled down on this kind of pointed storytelling. The network also tried to package itself as legitimate journalism, bringing on serious reporters like Shep Smith and Alisyn Camerota.

But the real stars were always its opinionated personalities, such as Sean Hannity and Bill O'Reilly. Journalism, as Camerota (now a CNN anchor) said in a panel that followed the New York City screening of the film, was not Ailes' central concern.

He was a "television wizard," she said, who was great at "sloganeering" and "messaging." As Camerota told me in an email, "in my experience, in a hundred conversations we had about work, he never discussed journalism -- never talked about the rules or the practice of it, which led to significant on-air mistakes. I often saw Roger break the rules of journalism."

Ailes promoted certain approaches to conservative politics that deeply impacted Trump, an avid Fox News viewer, whether he is conscious of them or not. The most obvious is that Ailes believed politicians had to shape their actions based on how they would play on television.

As the film shows, after Ailes first met Republican candidate Richard Nixon in 1968 during his appearance on the "Mike Douglas Show" (where Ailes was a producer), he explained to him that he would never win "without the skillful use of television."

As a Republican campaign consultant in the 1980s for numerous candidates, Ailes continued to impart this lesson on the GOP. He helped Mitch McConnell win a Senate seat in Kentucky, for instance, by humanizing his cold demeanor through campaign ads depicting him fishing (McConnell had no idea how to fish).

Meanwhile, Trump has a presidency that is tailor-made for television. As the film reminds us, Trump's regular appearances on Fox & Friends were the reason Republican primary voters could accept his candidacy in 2016. He has banked everything on tweets, rallies and provocations aimed at garnering attention on the network feed.

"Divide and Conquer" also shows that Ailes' chronic struggle with hemophilia led him to understand the power of fear in the public imagination. He was terrified of hurting himself and bleeding to death. He lived his entire life scared, seeing enemies all around, and he was prepared to strike hard against any potential threat. According to one high school friend interviewed in the film, "Roger's daily life was a fear of annihilation."

Ailes also loved to tell people a story about his father asking him to jump from the top of a bunk bed, and then not catching him -- the lesson being never trust anyone. The story may not be true, but Ailes telling it perfectly captured the way that he saw the world. Ailes understood that everyone had some fear of their own -- and he made that part of the way that Fox News told America what was happening in the world. The network garnered high ratings by stoking those anxieties and making certain the public kept tuning in because they were so terrified about what would come next.

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Trump has similarly trafficked in fear -- most recently playing up the threat posed by a caravan of "invaders," who, in reality, are largely poor Latin American migrants seeking better lives in the US.

Ailes was willing to do whatever was necessary to destroy his opponents and to maintain power. This, too, is quintessential Trump. His total war outlook was evident in the sleepy town of Cold Spring, New York, where Ailes and his wife lived. The town was trying to stop him from removing trees that blocked the scenic views from his mansion. Ailes went to war. He purchased the local newspaper, the Putnam County News, so that he could use it as a media bullhorn against local politicians. Ailes appeared at town hall meetings to rail against local officials and he threw his support behind campaigns to defeat officials who were against him.

This is the attitude that Trump brings to his presidency every day. He uses his bullhorn to tear down every opponent.

"Divide and Conquer" also makes it clear that Ailes' television baby was one that didn't take the feminist revolution of the 1970s very seriously. According to people interviewed for the film, the network treated female talent as sexualized objects. Ailes allegedly asked for sex in exchange for jobs, while the network required female correspondents to dress in provocative clothes and position themselves in certain ways to highlight their legs (a tactic he used in America's Talking, a failed cable network he launched before starting Fox). Ailes denied the charges against him.

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More recently, Trump has repeatedly made clear his disregard for the #MeToo movement, a modern manifestation of the feminist revolution.

Too often, Fox News is considered to be just one more channel in the universe of cable television. But the network has actually been a major force in American politics, and it has helped to reshape the modern Republican Party. The station has also insulated Trump from some of the fallout he otherwise would have experienced due to his radical style of governance and polarizing political views.

Ailes had a dream in 1968 when he first met Nixon -- and with the Trump presidency, that dream has come true. Trump has made certain that there is no daylight between the rhetoric and message of Fox News and the mainstream Republican Party.

The question for the GOP is whether the party is willing to continue paying the price? Former Fox personality Glenn Beck warns in the film, "It is easy to make somebody into a monster. It's hard to see that you're on that path, too."

The midterms exposed the limits of how much Fox News appeals to voters outside the narrowing Republican base. Many Americans don't subscribe to Ailes' vision of the world. Democrats now have the potential to render the world that Ailes created politically irrelevant.