Nielsen's exit is another victory for Trump adviser Stephen Miller

As news broke of the abrupt departure of Kirstjen Nielsen, Donald Trump’s second homeland security secretary, reports swiftly followed that the move had been orchestrated by Stephen Miller, one of the administration’s sharpest anti-immigration voices.

Nielsen’s resignation on Sunday, following months of speculation, cast a spotlight on the forces behind the president’s controversial policies on immigration, where Miller has been shown to be squarely at the center of influence.

The senior adviser and speechwriter is the architect of some of the most restrictive components of Trump’s immigration agenda.

Related: Spare us the self-serving excuses, Kirstjen Nielsen. You were the worst | Richard Wolffe

Bolstered by Trump’s frustration over promises including building a border wall and slashing legal immigration, Miller is poised to usher more hardliners into the administration, seeking to turn rhetoric into results.

But some immigration experts said the White House may soon learn that the challenge is not one of personnel, but one of policy and expectations.

“It’s fine for a White House adviser to have an outsize role in policy so long as there are demonstrable results,” said Rory Cooper, a Republican operative who played a central role in creating the homeland security department.

“But right now, we’ve got seeming chaos around the department emanating from the West Wing.”

Miller, 33, has been a prominent figure inside the White House since the first days of Trump’s presidency. He crafted the first iteration of Trump’s travel ban against Muslim-majority countries, which also indefinitely suspended the US refugee program, a move that sparked chaos and confusion at airports across the country.

A rebuke from the courts followed but Miller, who famously dismissed the poem at the base of the Statue of Liberty which welcomes migrants to American shores, was undeterred.

A native of Santa Monica, California, Miller was raised in a liberal household but forged a conservative identity at a young age. His writings as a college student at Duke University showed a disdain for multiculturalism and hardened a reputation as a rightwing ideologue.

Miller went on to work for the former Alabama senator Jeff Sessions, among the Republican party’s fiercest immigration hawks and who later served as Trump’s attorney general.

In 2013, from Sessions’ office, Miller fought against a bipartisan immigration reform bill that would have given a pathway to citizenship for the roughly 11 million undocumented migrants living in the US. Miller joined Trump’s 2016 campaign as a senior policy adviser and is one of the few such staffers still by the president’s side.

According to a report in Politico, Miller recently began berating mid-level officials at several federal departments and agencies, over rates of illegal immigration.

As the head of the Department of Homeland Security, Nielsen soon bore the blame for failing to stem the flow of migrants at the southern border. Having forcefully defended the Trump administration’s policy of separating families at the border last year, she insisted her hands were tied.

In the one spot where they were actually able to recruit strong people, it was the White House interference that was the problem.

Some of the president’s preferred approaches, such as reinstating family separations or shutting the border entirely, have invited legal scrutiny or threatened to derail the economy. Others have been rebuffed by Congress – such as funding for a border wall, which prompted Trump to lead the US into the longest government shutdown in history.

Senator Lindsey Graham, a close ally of the president, once complained that Miller’s presence in the White House made any immigration deal impossible.

“Every time we have a proposal, it is only yanked back by staff members,” Graham said last year. “As long as Stephen Miller is in charge of negotiating immigration, we’re going nowhere.”

Miller’s imprint on the Trump administration is so indelible that even members of his own family have distanced themselves from his worldview.

Related: 'You didn't get our message': key Trump aide Stephen Miller condemned by childhood rabbi

“I have watched with dismay and increasing horror as my nephew, an educated man who is well aware of his heritage, has become the architect of immigration policies that repudiate the very foundation of our family’s life in this country,” Miller’s uncle, Dr David S Glosser, wrote in an essay last year with reference to the family’s origins abroad.

Despite Trump’s tough talk on immigration, migrants fleeing violence and poverty in Central America have continued to seek refuge at America’s borders.

“Nothing they do at the border is going to stop a Honduran mother who wants to save her 13-year-old daughter, a Salvadoran family that wants to keep their 14-year-old boys out of the gangs, or a Guatemalan family who can’t feed his family unless he goes to America,” said Frank Sharry, the founder and executive director of America’s Voice, a progressive immigration reform group.

Nielsen’s dismissal had less to do with her leadership at the department, he noted, than it did Trump and Miller looking to pin the blame on others for the failure of their own strategy.

“They don’t have the levers to be as cruel and as mean and as lawless as they want to be,” Sharry said.

“They’re using a sledgehammer for something that requires a multi-pronged, complex strategy.”

Cooper said Trump’s looming re-election bid was probably driving the White House’s handling of the issue, particularly amid pressure from far-right supporters increasingly turned off by the administration’s inability to deliver.

“I think that they’re looking ahead at 2020 and realizing they’re vulnerable on their record as opposed to their rhetoric,” Cooper said.

“They’re speaking the language that some of their supporters want, but they’re not producing those results.”