Remember when the co-president was thought to be Elon Musk, before he decided to start a new political party because he was such a popular and charismatic figure in American politics? We were, apparently, quite wrong, as the real president was hiding in the shadows all along. He was a 39-year-old son of wealthy California Democrats whose formulative years were spent blaming janitors for not picking up the trash he tossed to the ground. And in the second Trump administration, he appears to have more power than pretty much anyone else. His name is Stephen Miller.

It was during his formative years that Mr. Miller developed a broader critique of society. He watched the left take over California and, in his view, turn it into a failed state — failures that he believed were directly attributable to immigration. As he explained years later, it was his experience in California that led him to conclude that “mass migration turns politics leftward” and that mass migration was turning the United States into California.

Unlike Trump, who held whatever views satisfied his need for self-aggrandizement at any given moment, Miller was an idealogue who knew what he hated. Immigrants. During the first Trump administration, Miller learned that he had a certain degree of influence with Trump and Bannon, but he lacked the power to make it happen.

In a relentless round of meetings, phone calls and emails, he reached deep into the federal bureaucracy and, according to a former Department of Homeland Security official, berated mid- and low-level bureaucrats inside the department. To keep their jobs, he told the officials, they needed to enforce a new policy that punished the families of undocumented immigrants by forcibly separating parents from their children.

Mr. Miller’s demands, however, went unmet. That’s because he was issuing them back in 2017, and the homeland security secretary, John Kelly, had issued his own edict to D.H.S. officials: If Mr. Miller ordered them to do something, they were to refuse, unless Mr. Kelly, the only one of the two men who’d been confirmed by the U.S. Senate to run the department, agreed to the order.

In Trump 2.0, Miller would not make the same mistake, holding sufficient sway to make sure no one appointed by Trump was competent and willing to act independently or follow the law.

Flash forward eight years, to this past May, when Mr. Miller, still livid and now the White House deputy chief of staff, paid a visit to the Washington headquarters of Immigration and Customs Enforcement, where he berated officials for not deporting nearly enough immigrants. He told the officials that rather than develop target lists of gang members and violent criminals, they should just go to Home Depots, where day laborers gather to be hired, or to 7-Eleven convenience stores and arrest the undocumented immigrants they find there.

This time, the officials did what Mr. Miller said. ICE greatly stepped up its enforcement operations, raiding restaurants, farms and work sites across the country, with arrests sometimes climbing to more than 2,000 a day.

But what of the Senate confirmed secretaries? Not much, apparently.

The crisis, from the immigration raids that sparked the protests to the militarized response that tried to put the protests down, was almost entirely of Mr. Miller’s making. And it served as a testament to the remarkable position he now occupies in Mr. Trump’s Washington. Kristi Noem, the homeland security secretary, who reportedly accompanied Mr. Miller on his visit to ICE headquarters, seems to defer to him. “It’s really Stephen running D.H.S.,” a Trump adviser said. The attorney general, Pam Bondi, is so focused on preparing for and appearing on Fox News that she has essentially ceded control of the Department of Justice to Mr. Miller, making him, according to the conservative legal scholar Edward Whelan, “the de facto attorney general.” And in a White House where the chief of staff, Susie Wiles, is not well versed or terribly interested in policy — “She’s producing a reality TV show every day,” another Trump adviser said, “and it’s pretty amazing, right?” — Mr. Miller is typically the final word.

Miller controls DHS? Makes some sense. Miller controls DoJ? No, he’s not a lawyer and, apparently, neither knows nor cares much more about law stuff than Bondi. Miller controls . . . Trump?

There is much truth to the conventional wisdom that the biggest difference between the first and second Trump presidencies is that, in the second iteration, Mr. Trump is unrestrained. The same is true of Mr. Miller. He has emerged as Mr. Trump’s most powerful, and empowered, adviser. With the passage of the big policy bill, ICE will have an even bigger budget to execute Mr. Miller’s vision and, in effect, serve as his own private army. Moreover, his influence extends beyond immigration to the battles the Trump administration is fighting on higher education, transgender rights, discrimination law and foreign policy.

While Trump is the front man, the dancing fool wearing a red baseball cap to match the red tie (who wears a baseball cap with a suit?), he’s neither a policy wonk nor ideologue. Indeed, beyond tariffs, which has been a Trump hobbyhorse for decades, it’s unclear whether he has any actual policy concerns at all. Policy, to Trump, is merely a mechanism to push his popularity with his MAGA base, and so he panders to their anger and throws them red meat as long as they respond with adoration. If he could get his puss on Mount Rushmore by doubling welfare, he would do that in a flash. It’s not his money, so what does he care?

But Miller? He cares. He cares a lot.

At the same time, Mr. Miller is a world-class brown noser. In an administration that puts a premium on sycophancy, he stands out for just how much he sucks up to his boss. “You have been watching the greatest economic master strategy from an American president in history,” Mr. Miller wrote on X shortly after Mr. Trump’s tariff flip-flop in April. Last year, when he was asked on a podcast to name his favorite ’80s action movie, he answered Jean-Claude Van Damme’s “Bloodsport,” an unusual choice — until you realize that Mr. Trump once deemed “Bloodsport” “an incredible, fantastic movie” and that he liked to watch it during flights on his private plane. The transition official told me that while it would overstate things to suggest that the president viewed Mr. Miller as indispensable — since no one in Mr. Trump’s circle ever is — Mr. Miller has been so central to Mr. Trump’s political operation for so long that the president would have a difficult time imagining what it would be like not to have Mr. Miller working for him.

It’s unclear whether Miller’s nose can maintain its hue long enough to survive Trump’s whimsy. After all, the winds of MAGA sometimes change and blow back on Trump, such as happened with his bombing of Iran and support of Israel, which has infuriated the Jew-haters and Hitler-lovers in his base. But unless and until that happens, the unelected and largely unknown “advisor” Stephen Miller will reshape the government in his image.

Last year, he reportedly quipped during a campaign meeting that if it was up to Mr. Miller, the population of the United States would be only 100 million people and they’d all resemble Mr. Miller. The humor, however, underscores something serious: On immigration, Millerism is a more consistent ideology than Trumpism.

Do you really want a nation where all the people resemble Miller?