Earlier this year, when Steve Bannon, Donald Trump’s chief strategist, was at the peak of his influence, I visited him at the White House. “This is totally top secret,” he said, ushering me inside his office on a Saturday night in mid-March. “This is like a vault.” The small room, shouting distance from the Oval Office, was open in the middle. Bannon, who was tieless and unshaven, said it wasn’t an office but “the war room,” and it was designed to be the battle station from which Bannon and his colleagues transferred the inchoate ideas of Candidate Trump into a more well-defined nationalism of President Trump.

“I took all the furniture out,” he said, pointing. “I put Scavino here—the Twitter guy.” He was referring to Dan Scavino, a former Trump golf caddy who directs social media for Trump, often skimming obscure message boards to find pro-Trump memes, such as the GIF the President tweeted of Trump body-slamming someone on whose face the CNN logo was superimposed.

One side of the war room is covered with sheets of paper listing the promises Trump made in a speech at Gettysburg in the final stretch of the campaign. When a promise is delivered, it gets crossed off and dated. “Hiring freeze on all federal employees to reduce federal workforce through attrition” had a large “X” through it and the date of “1/23.” On the opposite side of the wall was Bannon’s famous whiteboard, which listed dozens of Trump Administration policy goals and looked like Ted Kaczynski’s manifesto. “We keep a punch list,” Bannon said. “We’ve got every promise he’s ever made up here.”

Despite showing it off to a few other reporters and me, Bannon tried to keep the whiteboard’s contents secret. That plan was blown in April, when a visitor posted a picture of himself and Bannon in front of the whiteboard, and the board became a sensation online and among late-night comedians, generating dozens of memes and jokes. Trump was furious at the leak, perhaps because the writing was so messy, and the whiteboard was subsequently erased and redrawn with a cleaner and more professional look. (One new whimsical touch was the word “Paris” with a slash through it.)

Today, few of the big items on the Bannon whiteboard have been checked off and, in what seems like a monthly occurrence, anonymous whispers that Trump is planning to push Bannon out of the White House are circulating again. Trump has tired of reading about “President Bannon” and seeing his strategist’s face on the cover of Time. Bannon, in turn, dramatically reduced his public profile. (The Economist recently begged him to shoot his whiteboard for a cover story, and he declined.) Trump has refused to offer his support. “I like him. He’s a good man. He’s not a racist; I can tell you that,” Trump said at a press conference at Trump Tower, on Tuesday afternoon. “We’ll see what happens with Mr. Bannon.”

The possibility that Bannon may leave the White House raises the question of what Trumpism might look like without its leading architect in the West Wing. Last year, Bannon steered Trump to a narrow Electoral College victory by infusing Trump’s speeches with attacks on “international bankers” and “global financial powers” out to “plot the destruction of U.S. sovereignty” (language that the Anti-Defamation League believed was anti-Semitic) and betting on a surge of white working-class votes in the Midwest.

Defeating Clinton and the Democratic Party may have been the easy part. Bannon’s real goal, for many years, but especially since he took over Trump’s operation, has been to defeat the Republican Party. “If Trump wins, it literally changes everything,” Bannon told me last fall. “Because the Republican Party is never going back to what it once was.” The G.O.P. would become, as Trump himself started to say last year, “a workers’ party.” Bannon told me, “There’s a populist movement that demands that these trade deals are renegotiated. The banks are going to have to be broken up. There’s going to have be some sort of concern about workers’ wages and the middle class, Social Security, the entitlement programs. It’s going to be different.”

But the health-care fight showed how difficult it was to change the G.O.P. along the lines Bannon predicted. Trump himself—erratic, non-ideological, prone to taking the advice of the last person he talked to—was an unreliable vessel for Bannon’s brand of nationalism. In mid-January, the President-elect told the Washington Post that his health-care plan would be “insurance for everybody,” and indicated that he would fight the small-government wing of the G.O.P. “There was a philosophy in some circles that, if you can’t pay for it, you don’t get it. That’s not going to happen with us,” Trump told the paper. A few weeks later, Trump reversed course and backed a plan to repeal Obamacare, written by Paul Ryan, the Speaker of the House and leader of the modern G.O.P. that Bannon sought to blow up. If enacted, the plan would leave fourteen million more people without insurance next year and twenty-four million more in a decade, according to the nonpartisan Congressional Budget Office. The hardest hit would be white, older, rural, low-income voters—the very base that Trump had stolen from the Clintons.

Bannon continues to see himself as an outsider, even in the White House and the larger Republican Party. He views himself as surrounded by hostile forces in concentric circles: the moderating forces inside the White House, which, depending on the issue, include Gary Cohn, Jared Kushner, and H. R. McMaster, the media (“the opposition party”), and congressional Republicans. There are two streams of thought flowing from the vision of nationalism that Bannon is trying to foist on the G.O.P.: economic and xenophobic. Trumpism so far has been marked by the latter and very little of the former. Trump’s time in office, so far, will be remembered far more for his coddling of racist groups after the protests in Charlottesville than for any policy achievements. Whether or not Bannon remains in the White House, one clear test of whether Trumpism means anything more than the scapegoating of non-whites is whether the Republican Congress’s ambitious fall legislative agenda takes a more economically populist turn.

To be sure, though he enables and reinforces Trump’s ugly nativism, Bannon is hardly the deft, Rasputin-like manipulator he is seen as in American pop culture. He was adamantly opposed to the strike on Syria, and Trump ignored him. He has learned that Trump recoils from advisers who are seen to be controlling him. During the final fierce White House debate over whether to withdraw from the Paris climate treaty, Bannon tried not to talk to Trump for three days because he knew he was being inundated with advice—from C.E.O.s like Tim Cook and Elon Musk, from Rex Tillerson, from Ivanka and Jared—to stay in the treaty. In the end, Trump sided with the Bannon wing and withdrew from the accord, and Bannon learned an important lesson: for issues on which Trump is already on his side, blocking information from opponents can backfire.

Bannon’s big populist idea for the fall concerns taxes. Bannon has been deeply concerned that the health-care bill and most G.O.P. proposals on tax reform, including Trump’s official plan, would represent a huge tax cut for the wealthiest Americans. One idea he has floated inside the White House is creating a new, higher tax rate, perhaps for income over a million dollars. He believes that the secret to sell it is to convince the President that raising taxes is a political winner. Bannon has become friends with Gerald Butts, a longtime political adviser to the Canadian Prime Minister, Justin Trudeau. They met in New York during the transition and now talk regularly. Bannon sees Butts as a sort of left-wing version of himself. Last year, as the Prime Minister’s popularity was in decline, Trudeau pushed through a tax hike on the rich, and it helped him rebound.