WASHINGTON — It was almost noon Friday when a Marine suddenly appeared outside the West Wing doors, a sign that President Donald Trump had belatedly reported to the Oval Office.

For the third day in a row, the president had been in the White House residence all morning, fuming about federal investigations that have moved closer to him — and are likely to get worse.

His former confidant, attorney Michael Cohen, and other once-stalwart supporters have flipped, becoming witnesses for a Justice Department he has struggled to bend to his will. Prosecutors also secured the cooperation of American Media Inc., the tabloid publisher that routinely helped Trump muzzle bad stories and target his enemies.

The result is Trump has become increasingly isolated as he enters what may be the most difficult stretch of his presidency, one laden with political and legal dangers.

Come January, a newly elected Democratic majority in the House is expected to issue a blizzard of subpoenas. Rep. Nancy Pelosi, who is expected to be the next House speaker, said Thursday that a House committee will probably “take the first steps” toward seeking Trump’s long-hidden tax returns, an effort he will almost certainly fight.

Special counsel Robert Mueller continues to investigate Russia’s role in Trump’s election and whether the president sought to obstruct justice by trying to end the inquiry.

“I don’t think he’s ever in his life been in this position,” said Barbara Res, who worked years for the Trump Organization, the president’s private family-run business.

Trump already is struggling to exert his influence. Few Republicans have endorsed his threats to force a government shutdown over Christmas if Congress doesn’t approve $5 billion for a proposed border wall.

On Thursday, the Republican-controlled Senate took a direct slap at the White House, voting unanimously by voice vote to condemn Saudi Crown Prince Mohammed bin Salman for the brutal slaying of journalist Jamal Khashoggi. Trump has publicly backed the crown prince despite U.S. intelligence findings that he masterminded the killing.

Tony Schwartz, the ghost writer of Trump’s 1987 best-selling book, “The Art of the Deal,” said the reckoning Trump faces follows decades of operating under a belief that he was above the law.

“He got away with so much, for so long, that he came to believe he was untouchable and invincible,” Schwartz said.

Several others close to the president, granted anonymity to speak openly about conversations with him, said Trump already senses diminishing respect and worries about losing support from powerful financial donors and Republican lawmakers as his legal and political troubles worsen.

“They’re still not saying it publicly, but most Republicans on the Hill understand ... that it’s not going to end well, that it’s going to be bad,” said a longtime Republican operative close to party leadership.

Trump has long railed against “rats” who cooperate with law enforcement.

In a Twitter message Thursday, he accused Cohen — who was sentenced to three years in federal prison the day before for tax fraud, campaign finance violations and other crimes — of pleading guilty “in order to embarrass the president and get a much reduced prison sentence, which he did.”

But the most pressing threat may be the investigation, led by the U.S. attorney’s office in Manhattan, in what prosecutors say was an illegal scheme to pay $280,000 to two women shortly before the 2016 election to buy their silence about alleged sexual affairs with Trump years earlier.

Cohen directly paid Stormy Daniels, a porn star, and was reimbursed by the Trump Organization. American Media Inc., publisher of the National Enquirer, paid Karen McDougal, a former Playboy model, for the rights to her story and then never ran it.

Prosecutors say Trump directed both payments, which were illegal because they were intended to influence the election and functioned as excessive campaign contributions that were not disclosed.

In an interview aired by ABC’s “Good Morning America” on Friday, Cohen said Trump “of course” knew the payment was wrong. “Nothing at the Trump Organization was ever done unless it was run through Mr. Trump,” Cohen said.

Establishing whether Trump knew the hush money would violate campaign finance laws is a key hurdle for prosecutors if they intend to charge him.

The president once denied knowing about the payments, but he abandoned that as evidence showed otherwise.

He now argues that he never asked Cohen to break the law.

Some of Trump’s Republican allies have begun to publicly admit concerns about whether Trump violated the law in the hush money scheme — even if he’s unlikely to face prosecution while in office.

“Am I concerned that the president might be involved in a crime? Of course,” Sen. Bill Cassidy, R-La., told reporters, although he also expressed doubt about whether the violation amounts to a crime.

There are more signs of trouble ahead.

With the White House chief of staff, John Kelly, leaving at the end of the month, Trump has struggled to find a replacement.

After Nick Ayers, a top aide to Vice President Mike Pence, turned him down, several others reportedly under consideration also said no thanks.

On Friday, Trump announced via Twitter that budget director Mick Mulvaney will serve as acting chief of staff.

While his difficulty filling the job full-time highlights the president’s isolation, Schwartz said Trump almost certainly doesn’t agree.

“He doesn’t feel the ordinary human emotions most of us do,” he said. “For example, he’s free of shame because he has no conscience, nor any self-awareness. So while his behavior certainly suggests he feels increasing fear and isolation, he would be the last one to know it.”