Last month, when President Donald Trump toured a Boeing aircraft plant in North Charleston, South Carolina, he saw a familiar face in the crowd that greeted him: Patrick Caddell, a former Democratic political operative and pollster who, for forty-five years, has been prodding insurgent Presidential candidates to attack the Washington establishment. Caddell, who lives in Charleston, is perhaps best known for helping Jimmy Carter win the 1976 Presidential race. He is also remembered for having collaborated with his friend Warren Beatty on the 1998 satire “Bulworth.” In that film, a kamikaze candidate abandons the usual talking points and excoriates both the major political parties and the media; voters love his unconventionality, and he becomes improbably popular. If the plot sounds familiar, there’s a reason: in recent years, Caddell has offered political advice to Trump. He has not worked directly for the President, but at least as far back as 2013 he has been a contractor for one of Trump’s biggest financial backers: Robert Mercer, a reclusive Long Island hedge-fund manager, who has become a major force behind the Trump Presidency.

During the past decade, Mercer, who is seventy, has funded an array of political projects that helped pave the way for Trump’s rise. Among these efforts was public-opinion research, conducted by Caddell, showing that political conditions in America were increasingly ripe for an outsider candidate to take the White House. Caddell told me that Mercer “is a libertarian—he despises the Republican establishment,” and added, “He thinks that the leaders are corrupt crooks, and that they’ve ruined the country.”

Trump greeted Caddell warmly in North Charleston, and after giving a speech he conferred privately with him, in an area reserved for V.I.P.s and for White House officials, including Stephen Bannon, the President’s top strategist, and Jared Kushner, Trump’s son-in-law. Caddell is well known to this inner circle. He first met Trump in the eighties. (“People said he was just a clown,” Caddell said. “But I’ve learned that you should always pay attention to successful ‘clowns.’ ”) Caddell shared the research he did for Mercer with Trump and others in the campaign, including Bannon, with whom he has partnered on numerous projects.

The White House declined to divulge what Trump and Caddell discussed in North Charleston, as did Caddell. But that afternoon Trump issued perhaps the most incendiary statement of his Presidency: a tweet calling the news media “the enemy of the American people.” The proclamation alarmed liberals and conservatives alike. William McRaven, the retired Navy admiral who commanded the 2011 raid that killed Osama bin Laden, called Trump’s statement a “threat to democracy.” The President is known for tweeting impulsively, but in this case his words weren’t spontaneous: they clearly echoed the thinking of Caddell, Bannon, and Mercer. In 2012, Caddell gave a speech at a conference sponsored by Accuracy in Media, a conservative watchdog group, in which he called the media “the enemy of the American people.” That declaration was promoted by Breitbart News, a platform for the pro-Trump alt-right, of which Bannon was the executive chairman, before joining the Trump Administration. One of the main stakeholders in Breitbart News is Mercer.

Mercer is the co-C.E.O. of Renaissance Technologies, which is among the most profitable hedge funds in the country. A brilliant computer scientist, he helped transform the financial industry through the innovative use of trading algorithms. But he has never given an interview explaining his political views. Although Mercer has recently become an object of media speculation, Trevor Potter, the president of the Campaign Legal Center, a nonpartisan watchdog group, who formerly served as the chairman of the Federal Election Commission, said, “I have no idea what his political views are—they’re unknown, not just to the public but also to most people who’ve been active in politics for the past thirty years.” Potter, a Republican, sees Mercer as emblematic of a major shift in American politics that has occurred since 2010, when the Supreme Court made a controversial ruling in Citizens United v. Federal Election Commission. That ruling, and several subsequent ones, removed virtually all limits on how much money corporations and nonprofit groups can spend on federal elections, and how much individuals can give to political-action committees. Since then, power has tilted away from the two main political parties and toward a tiny group of rich mega-donors.

Private money has long played a big role in American elections. When there were limits on how much a single donor could give, however, it was much harder for an individual to have a decisive impact. Now, Potter said, “a single billionaire can write an eight-figure check and put not just their thumb but their whole hand on the scale—and we often have no idea who they are.” He continued, “Suddenly, a random billionaire can change politics and public policy—to sweep everything else off the table—even if they don’t speak publicly, and even if there’s almost no public awareness of his or her views.”

Through a spokesman, Mercer declined to discuss his role in launching Trump. People who know him say that he is painfully awkward socially, and rarely speaks. “He can barely look you in the eye when he talks,” an acquaintance said. “It’s probably helpful to be highly introverted when getting lost in code, but in politics you have to talk to people, in order to find out how the real world works.” In 2010, when the Wall Street Journal wrote about Mercer assuming a top role at Renaissance, he issued a terse statement: “I’m happy going through my life without saying anything to anybody.” According to the paper, he once told a colleague that he preferred the company of cats to humans.

Several people who have worked with Mercer believe that, despite his oddities, he has had surprising success in aligning the Republican Party, and consequently America, with his personal beliefs, and is now uniquely positioned to exert influence over the Trump Administration. In February, David Magerman, a senior employee at Renaissance, spoke out about what he regards as Mercer’s worrisome influence. Magerman, a Democrat who is a strong supporter of Jewish causes, took particular issue with Mercer’s empowerment of the alt-right, which has included anti-Semitic and white-supremacist voices. Magerman shared his concerns with Mercer, and the conversation escalated into an argument. Magerman told colleagues about it, and, according to an account in the Wall Street Journal, Mercer called Magerman and said, “I hear you’re going around saying I’m a white supremacist. That’s ridiculous.” Magerman insisted to Mercer that he hadn’t used those words, but added, “If what you’re doing is harming the country, then you have to stop.” After the Journal story appeared, Magerman, who has worked at Renaissance for twenty years, was suspended for thirty days. Undaunted, he published an op-ed in the Philadelphia Inquirer, accusing Mercer of “effectively buying shares in the candidate.” He warned, “Robert Mercer now owns a sizeable share of the United States Presidency.”

Nick Patterson, a former senior Renaissance employee who is now a computational biologist at the Broad Institute, agrees that Mercer’s influence has been huge. “Bob has used his money very effectively,” he said. “He’s not the first person in history to use money in politics, but in my view Trump wouldn’t be President if not for Bob. It doesn’t get much more effective than that.”

Patterson said that his relationship with Mercer has always been collegial. In 1993, Patterson, at that time a Renaissance executive, recruited Mercer from I.B.M., and they worked together for the next eight years. But Patterson doesn’t share Mercer’s libertarian views, or what he regards as his susceptibility to conspiracy theories about Bill and Hillary Clinton. During Bill Clinton’s Presidency, Patterson recalled, Mercer insisted at a staff luncheon that Clinton had participated in a secret drug-running scheme with the C.I.A. The plot supposedly operated out of an airport in Mena, Arkansas. “Bob told me he believed that the Clintons were involved in murders connected to it,” Patterson said. Two other sources told me that, in recent years, they had heard Mercer claim that the Clintons have had opponents murdered.