WASHINGTON — Those who love and fear President-elect Donald Trump agree on one thing: He is bent on upending nearly every aspect of the presidency.

But as he takes the oath of office Friday, Trump's mythology will begin to meet reality.

And the debate has already begun over which elements of Trumpism will be truly revolutionary and which will simply represent a break from his party or a hard turn from President Barack Obama.

Those questions will be central to Trump's administration, as factions within his circle and Republican leaders in Congress vie for influence while Trump confronts Washington traditions that have evolved to withstand dramatic change.

The most radical aspect of Trump's presidency is likely to be the man occupying it. He has flouted ethics standards, refused to release tax returns, bashed the intelligence community, maligned the press, challenged facts and communicated unlike anyone who has held the office.

The policy arena is less certain to see unprecedented change. Some of his most polarizing proposals, such as building a wall on the Mexican border, may be extremely controversial yet fall loosely within current policy. President George W. Bush signed a law in 2006, with Democratic support, that authorized construction of 700 miles of “reinforced fencing” along the border, though it was never fully funded.

Trump's promises to end trade deals and negotiate lower drug prices fall into another category, in that they contradict decades of GOP economics. Yet neither idea falls outside the typical boundaries of policy debate.

But if Trump chooses to ban Muslims from entering the country, or creates a religious-based registry, that would certainly qualify as radical. And Trump's suggestion during the campaign that more allied countries might build nuclear weapons to defend themselves could also put him in uncharted territory.

Other possibilities raised during his campaign and transition, such as dropping the ban on waterboarding and reconfiguring post-World War II alliances in Europe, may also qualify as tectonic shifts in the world order, but are nonetheless resonant of an earlier era.

Though Trump would face strong resistance from the intelligence, foreign policy and military establishments by taking either of those actions, nations do shift alliances, and waterboarding, while condemned by human rights groups and outlawed in 2015, was used covertly on captives during the Bush administration.

Trump's impact on policy will take time to discern.

Trump and his allies have been inconsistent on which ideas he would pursue. But they have stood by the notion that Trump will not be bound by traditional rules.

Former House Speaker Newt Gingrich, a close adviser during the campaign, said the establishment should not assume that Trump and his team will be cowed by their move to the Oval Office.

“The city keeps hoping that they won't be the guys who won the election,” Gingrich said. “And I think the city's going to be disappointed.”

More certain is Trump's personal impact on the office of the presidency.

Historians are hard-pressed to find anyone who has come into office quite like Trump, an outsider who is filling his Cabinet with others who also lack government experience. Some of those picks have shown disdain or outright hostility for the agencies they will run.

Trump has forgone the usual attempts to reach out to opponents, instead using Twitter to lash out at enemies both large and small, inflame racial tensions and weigh in casually on risky topics such as nuclear weapons.

It has prompted concern from global allies and record-low approval ratings that could affect his ability to rally the public.

“He's like the mountain goat who jumps from peak to peak in areas where the president has no responsibility, such as the Golden Globe Awards,” said Stephen Hess, who served on the White House staff for Presidents Dwight Eisenhower and Richard Nixon and as an adviser to Gerald Ford and Jimmy Carter. “We're testing America's traditional notion of presidents or how presidents act.”

Other presidents have orchestrated dramatic shifts in foreign policy. Yet they have rarely attempted to do so before taking office, and never through a combination of phone calls and tweets delivered without consulting experts at the State Department.

Trump's former campaign manager, Corey Lewandowski, pointed to two forms of Trump outreach that have rattled norms. One was his call with Taiwanese President Tsai Ing-wen, which provoked authorities in China and alarmed foreign policy experts.

The other is his use of Twitter, phone calls and personal meetings to pressure individual companies over their foreign hiring practices or government contracts.

Trump used the interactions to sell his message, but drew criticism from economists who say a president does not have time to make significant change one business at a time.

Trump won wider praise when he used Twitter to lobby fellow Republicans in Congress to keep a House ethics office that some were eager to gut.

Gingrich said Trump will deal with government the way Elon Musk, who created a privately financed space program, challenged NASA's bureaucracy. Americans elected Trump because the system is “decaying dramatically,” Gingrich said.

“It's way too bureaucratic. It's way too dishonest. It's way too politically correct,” he said. “It needs somebody like this to reset the game.”

Yet many who have worked and studied government warn that Trump's lack of experienced hands around him will hamper his ability to reset the system, given its complexity.

Changing the rules all at once, as Trump is trying to do, creates more shock than government can absorb, said John Hudak, deputy director of the Center for Effective Public Management at the Brookings Institution.

He compared it to showing up at Trump Tower with no real estate experience and declaring to the president-elect that he planned to overhaul the rules of the industry in one swoop.

“He's going to laugh me right out the door,” he said.

noah.bierman@latimes.com