Washington (CNN) As President Donald Trump pushed his administration in recent weeks to take the drastic step of shutting down the US southern border, one key adviser fed Trump's impulse: Stephen Miller, the President's 33-year-old senior policy adviser and chief speechwriter who has long channeled Trump's hardline immigration views.

As most of the President's advisers sought to dissuade him from the idea, warning the economic consequences would be devastating, Miller encouraged the President to move forward with shutting down some ports of entry, two administration officials said.

While Trump was ultimately convinced to back down, Miller's stance solidified his position in Trump's mind as the chief facilitator of the President's hardline instincts and appears to have won him more power.

In the wake of that debate, Trump has moved to more formally empower Miller, telling aides in a recent Oval Office meeting that he was putting Miller in charge of all immigration and border issues, according to a person familiar with the meeting.

In the days since, there has been a cascade of personnel moves inside the Department of Homeland Security. Last week, Miller convinced the President to pull the nomination of Ron Vitiello as the next director of Immigration and Customs Enforcement, bluntly telling Trump that Vitiello was "soft" on closing the southern border. The decision blindsided department officials and Republican lawmakers, who initially thought it was a clerical error.

Two days later, Trump announced the forced resignation of Homeland Security Secretary Kirstjen Nielsen. Two days after that, Nielsen's deputy, Claire Grady, resigned under pressure to make room for the President's preferred pick for acting secretary, Kevin McAleenan, the Customs and Border Patrol Commissioner.

More turnover is anticipated -- giving Miller an expansive foothold to shape the administration's immigration policies and attempt end-runs around current laws.

More than anyone in the West Wing, Miller has developed a mind-meld with the President on immigration, arguably his most important issue heading into the 2020 election. Where some advisers have sought to temper Trump's instincts, Miller has unabashedly stoked them, earning him a reputation around Washington as a radical and influential voice inside the White House.

Administration officials caution that Miller is not some all-powerful and invisible hand driving every one of Trump's actions on immigration, as he is sometimes made out to be. The reality, they say, is more complex. While Miller goaded Trump on as he contemplated shutting down the US-Mexico border, the idea originated with Trump. And other advisers, including White House chief of staff Mick Mulvaney as well as senior adviser and Trump son-in-law Jared Kushner, hold sway with Trump on immigration matters.

Asked Wednesday morning about Miller's role, the President praised Miller, but sought to dispel any questions about who is really driving his immigration policy.

"Frankly, there's only one person that's running it. You know who that is? It's me," he said, pointing to his head.

Minutes later, Miller responded to CNN's request for comment from the previous day: "Only one person in this U.S. government runs immigration policy: President Donald Trump."

Still, Miller now finds himself in the driver's seat and better positioned than ever before to shape the administration's immigration and border security policies. The question is what he will do with the newfound position of authority. Will he be able to deliver for Trump, or, like so many before him, be the one to bear the blame for a failure to overcome intractable issues and legal hurdles?

Former Sessions staffer

Before he entered Trump's universe, Miller was communications director to then-Sen. Jeff Sessions of Alabama. Their hardline views on immigration often left them on the edges in the Senate, fighting broader bipartisan immigration reform efforts every step of the way.

"All I can think of is his non-stop emails telling us Jeff Sessions was coming to the floor to talk about some fringe thing the broader party disagreed with," one former GOP aide said, chuckling. "Now he's the mainstream in this White House and he's got more power than he could've ever dreamed of."

Miller's path to power began when he joined the still fledgling operation working to elect Trump in January 2016 as a policy adviser and speechwriter. He quickly developed a knack for channeling Trump's voice on the stump and became a core member of Trump's inner circle of aides.

After Trump was inaugurated, it took Miller less than a week to make his mark in the West Wing. He crafted an executive order alongside then-chief strategist Steve Bannon to ban travelers to the US from seven Muslim-majority countries -- a move that blindsided even some members of Trump's Cabinet.

Since then, Miller has influenced the administration's border and immigration policies and crafted the rhetoric to go along with it. Last summer, he played a key role in advocating for the administration's "zero tolerance" family separation policy. And he was instrumental in insisting the White House not budge in its demands for border wall funding before and during the government shutdown this winter.

Miller has at times pushed the bounds of his immigration authority. In the past, he attempted to direct then-Homeland Security Secretary John Kelly and Nielsen to take certain immigration actions, administration officials said.

"Kelly would say yeah, no, I'm the secretary -- that's not how this works," one administration official recalled.

In recent weeks Miller has ramped up his calls to mid-level officials at the Department of Homeland Security, administration officials said, berating them for not doing more to stem spiking immigration numbers at the border and directing them to take specific actions.

Vitiello, the ICE nominee, isn't the only official Miller has tried to push out. He has also openly complained about the department's general counsel, John Mitnick, and told Trump that L. Francis Cissna, the head of United States Citizenship and Immigration Services, has proven unable to deliver results. While Miller played a critical role in ousting Nielsen -- often complaining about her to and with Trump -- she had few allies in the West Wing to begin with.

View from Capitol Hill

On Capitol Hill, Miller is equal parts agitator and close ally. Republicans who have worked with him on immigration, particularly those supportive of his views on the issue, note what they perceive to be an encyclopedic knowledge of immigration laws -- a resource in the heated debates that have transpired with Democrats. But it can cut both ways, one senior GOP aide said: "He remembers everything your boss has said or done on the issue. And he's got the goods to kill you on it if you take a new position that runs contrary to something you've proposed or voted on before."

Miller drew the ire of a number of Senate Republicans in 2018 for his perceived role in blowing up a bipartisan immigration deal that would have granted legal status to undocumented immigrants who crossed into the US as children.

"As long as Stephen Miller is in charge of negotiating immigration, we are going nowhere," Sen. Lindsey Graham of South Carolina told reporters at the time.

The debate, and legislative implosion that followed, served as a window for some GOP senators into how Miller operates. "You didn't necessarily see him or hear from him directly," said one senator involved in the bipartisan talks. "But you knew he was always there, always working, and always the last person in the president's ear."

As to the most striking part of the process? "He wouldn't give an inch," the senator said.

Miller's efforts to gut DHS's current leadership structure became the latest flashpoint between Miller and lawmakers -- with Republican senators in particular striking out at Miller.

Iowa Sen. Chuck Grassley told the Washington Post that "it would be hard for (Miller) to demonstrate he's accomplished anything for the President." And South Dakota Sen. John Thune, the Republican majority whip, said amid the shakeup, he hopes there will be a check on Miller's influence.

"I do think his voice is influential and I hope it gets balanced out with other voices there," Thune told CNN.

Miller's status among Democrats is so baked in by this point that one top administration official attempted to make light of it.

During a meeting of bipartisan congressional staffers convened by Vice President Mike Pence during the government shutdown, two sources said, Miller walked through a series of potential deal points. When he was done, Kushner looked at the Democratic staffers in the group.

"See? He's not so bad!" Kushner exclaimed.

There were a few chuckles, the sources said, but mostly the Democrats in attendance shifted uncomfortably in their seats.

White House operator

White House aides describe Miller as a subtle operator inside the West Wing. Rather than fighting it out with other aides in meetings with Trump, Miller tends to use his one-on-one moments with Trump to shape the President's thinking.

Stephen Miller looks on as President Trump hosts a discussion on border security

"I think the thing people don't fully understand about Miller is that he works in the shadows," one former senior White House official said. "He almost never speaks during meetings with POTUS when other staff members are there."

There are other times though, when Miller is not so subtle.

During one meeting on border security last fall, Shahira Knight, Trump's legislative affairs director, was arguing that the administration should wait until after the midterm elections to push for border security funding, since it would be in a stronger position to do so. Miller cut her off mid-sentence, two people in the room said, shooting back a list of reasons why the administration would almost certainly be in a weaker position after the midterms, including that the Republicans were guaranteed to lose seats and possibly control of the House.

His outburst silenced the room, the two people said. One staffer, who asked to remain anonymous to discuss the interaction, described it as an "evisceration."

For Knight, whose job is to maintain the White House's relationship with lawmakers, Miller hasn't exactly been helpful, according to several people familiar with the situation.

Miller has avoided making enemies inside the White House, where his colleagues often tout his wry sense of humor. As one of the longest-tenured members of the President's inner circle, Miller has deftly moved between warring factions in a West Wing famous for back-biting. Early in the administration, he worked closely with Bannon to implement hard-right nationalist policies, including the travel ban. But as Bannon's fortunes began to turn , Miller sought to ingratiate himself with Kushner, who had had long been feuding with Bannon.

"Your polling numbers are actually very strong considering Steve won't stop leaking to the press and trying to undermine Jared," Miller told the President when he thought they were alone, according to former White House aide Cliff Sims' book "Team of Vipers." "If Steve wasn't doing that, I bet you'd be ten points higher."

Miller has also forged a close working relationship with Mulvaney. When Trump kept pushing for border closures, it was Miller and Mulvaney who huddled together to brainstorm how Trump could close parts of the border without devastating the economy. They suggested only closing certain ports of entry, stalling some trade instead of all and slow-walking entries into the US.

Miller and Mulvaney were also on the same side during the debate over whether Trump should declare a national emergency. Instead of telling Trump why he shouldn't, both encouraged him to do so and looked for ways to make it work. Just as Miller still operates with the same fringe tendencies he honed working for Sessions, Mulvaney still makes decisions like he's a member of the Tea Party, sources said.

Can he deliver?

Some of the changes Trump has sought, such as denying migrants the ability to claim asylum once they cross into the US, remain illegal. And Miller will be constrained by those same laws.

"At the end of the day," a senior administration official said, "the President refuses to understand that the Department of Homeland Security is constrained by the laws."

But Miller is already working with other officials to develop workarounds to current laws, including developing policies to make it more difficult for people to obtain asylum protections by cracking down on the number of people deemed to have a "credible fear" of returning to their home country.

"The biggest issue is getting a hold of the credible fear system," a senior administration official said, arguing there is a "cultural bias" resulting in a high number of credible fear threats being approved. "The purpose of the credible fear threat is not to ease path of meritless asylum seekers but to weed them out."

Mark Krikorian, the executive director of the Center for Immigration Studies, a group that supports reduced immigration to the US, said he believes there are steps the Trump administration could still take to address the overcapacity situation at immigration detention centers and cut down on the number of border crossings.

But even though Krikorian said he believes empowering Miller will be a boon to his and the Trump administration's policy goals, he acknowledged limits to the impact Miller can have.

"There's no question that there's a limit to what they can do without changes from Congress. The question is how realistic are the President's expectations? Is he expecting them to solve this problem or is he expecting them to make some progress so that he can then go to the public and to Congress and say Congress has to do its part too?" Krikorian said. "That's kind of up to the President. That's not really something Stephen can do."