After weeks of dismissing the impeachment inquiry as a hollow partisan attack, President Trump and his closest advisers now recognize that the snowballing probe poses a serious threat to the president — and that they have little power to block it, according to multiple aides and advisers.

The dawning realization comes as Democrats rapidly gather evidence from witness after witness testifying about the pressure put on Ukraine to investigate Trump’s political rivals. The president is increasingly frustrated that his efforts to stop people from cooperating with the probe have so far collapsed under the weight of legally powerful congressional subpoenas, advisers said.

The Democratic strategy got a boost Friday from a federal judge, who ruled that the House impeachment inquiry is legal. In the coming week, House investigators are scheduled to hear testimony from five more witnesses, including on Saturday from an acting assistant secretary of state for Europe, who is expected to testify about the efforts to oust the previous U.S. ambassador.

Democratic and Republican lawmakers addressed how a group of GOP members barged into a secure hearing room on Oct. 23. (The Washington Post)

In a sign of the growing realization of his potential jeopardy, Trump has brought back Jane and Marty Raskin, criminal defense attorneys who were part of his legal team during the Mueller investigation, to help him navigate the impeachment inquiry, along with his attorney Jay Sekulow and White House lawyers. Their return is a late acknowledgment, some White House advisers say, that the facts coming out are bad for the president and that both his White House and personal attorneys need to try to get in front of what else may emerge.

The president’s reconstituted legal team is racing to master details about the administration’s dealings with Ukraine, along with the efforts of their longtime co-counsel, Rudolph W. Giuliani, to push Ukraine officials to investigate Trump’s Democratic rivals.

Meanwhile, White House officials have begun holding regular impeachment strategy meetings, often in the Situation Room. Some advisers are discussing bringing in a veteran lawyer with impeachment experience and actively seeking a communications strategist, according to advisers and officials.

Sen. Lindsey O. Graham (R-S.C.) introduced a resolution denouncing the impeachment inquiry into President Trump in a news conference on Oct. 24. (The Washington Post)

The White House did not respond to requests for comment. Sekulow and the Raskins declined to comment.

The belated scramble — a month after the House formally launched its impeachment inquiry — serves as a recognition that the White House’s strategy of refusing to cooperate with the probe has failed to stymie it, according to Trump advisers and people involved in responding to House requests.

That posture was driven by Trump, who dictated much of a defiant letter sent by White House Counsel Pat Cipollone to House leaders earlier this month that claimed the inquiry was constitutionally invalid, according to people familiar with his role. They, like others in this story, spoke on the condition of anonymity to describe private White House conversations.

That has put administration officials called to appear on the Hill in a box. Late Friday, Charles Kupperman, who served as a deputy to former national security adviser John Bolton, went to court to ask a federal judge how to resolve the conflicting orders he faces: one from Congress demanding his testimony, and the other from the White House claiming he has testimonial immunity and instructing him not to appear. Kupperman, who is seeking an expedited ruling, is represented by attorney Charles Cooper, who is also serving as a lawyer for Bolton.

Although Democrats have not yet been able to secure the participation of all the witnesses they have sought, nine key figures have testified so far, including two current ambassadors and a Pentagon official.

That’s largely because attorneys for officials who have been called for depositions concluded that the White House’s legal arguments are weak compared with a congressional subpoena, according to people familiar with the conversations.

While senior White House advisers have a reason to claim they legally can ignore a congressional subpoena because of executive privilege concerns, that is not the case with career employees, legal experts said.

As a result, civil servants within the bureaucracy that Trump decries as the Deep State are giving Congress a series of damaging accounts about the extent of the effort to pressure Ukraine.

“These are civil servants who realize their duty is public service and honoring the Constitution . . . and [some] are deeply disturbed by the horrendous abuses they have witnessed,” said Bruce Freed, president of the Center for Political Accountability, a nonprofit that promotes transparency in government and campaigns.

A number of outside advisers are perplexed that the White House hasn’t filed an injunction or taken some other legal step to stop the parade of officials sharing what they saw and heard.

“Many Trump allies are concerned and don’t understand the strategy of not filing an injunction,” said Jason Miller, a Trump ally. “It’s a head-scratcher. President Trump and the administration have clearly said they don’t want folks participating in this sham process and stepping all over presidential privilege.”

The success the Democrats have had in securing powerful witness testimony is a stark contrast with their efforts earlier in the year, when they sought to interview former White House advisers about Trump’s efforts to block the Russia investigation.

At the time, the White House invoked the risks to executive privilege — directing witnesses such as former White House counsel Donald McGahn and former communications director Hope Hicks that they could not share their accounts with Congress without jeopardizing confidential conversations they had with the president.

The assertion wasn’t settled law, but it worked: Despite a subpoena, McGahn declined to testify, and Hicks agreed to appear in a closed-door session accompanied by White House lawyers but then declined to answer questions about her time in the White House.

In the impeachment inquiry, the administration has sought to use that same playbook, warning witnesses that they should not participate, according to White House talking points and letters sent by top agency officials to their employees.

Cipollone’s main argument: The House impeachment inquiry was not legally “authorized” by a House vote, and so the administration was not required to participate.

“Your inquiry is constitutionally invalid and a violation of due process,” he wrote in his Oct. 8 letter to House leaders. “For the foregoing reasons, the President cannot allow your constitutionally illegitimate proceedings to distract him and those in the Executive Branch from their work on behalf of the American people.”

However, the House can make its own rules and can conduct investigations under its own terms, legal experts said. On Friday, Chief U.S. District Judge Beryl A. Howell of Washington dismissed arguments by Republicans that the House must first vote to authorize an impeachment inquiry, calling the notion politically “appealing” but legally “fatally flawed.”

“No governing law requires this test — not the Constitution, not House Rules, and not [the grand jury secrecy rule], and so imposing this test would be an impermissible intrusion on the House’s constitutional authority,” Howell wrote.

White House lawyers also have argued that witnesses should not testify unless an administration lawyer is present — an argument the White House made in talking points distributed to Republicans when ousted ambassador Marie Yovanovitch was facing requests from the House.

Without a government lawyer by her side, the White House claimed, “there is serious danger that she could breach her obligations as a current employee not to reveal such information without authorization.”

The House Intelligence Committee responded with a subpoena, saying “the illegitimate order from the Trump Administration not to cooperate has no force.”

Yovanovitch complied and laid out in her testimony details about her abrupt ouster from Ukraine.

“Today, we see the State Department attacked and hollowed out from within,” she said in prepared remarks obtained by The Washington Post, warning that U.S. adversaries such as Russia stand to benefit “when bad actors in countries beyond Ukraine see how easy it is to use fiction and innuendo to manipulate our system.”

A similar effort to prevent this week’s testimony of Deputy Assistant Secretary of Defense Laura Cooper also failed. In a letter to her attorney, the Pentagon urged her not to participate in the inquiry, according to a copy obtained by The Post.

“This letter informs you and Ms. Cooper of the Administration-wide direction that Executive Branch personnel ‘cannot participate in [the impeachment] inquiry under these circumstances,’ ” Deputy Defense Secretary David Norquist wrote. “In the event that the Committees issue a subpoena to compel Ms. Cooper’s appearance, you should be aware that the Supreme Court has held . . . that a person cannot be sanctioned for refusing to comply with a congressional subpoena unauthorized by House Rule or Resolution.”

She nevertheless showed up to give testimony. Her interview was delayed for hours when GOP lawmakers barged into the deposition room, a tactic encouraged by Trump, according to people familiar with his role.

As the inquiry has plowed forward, Trump has expressed anger at the number of people who are testifying, asking why they can’t be stopped, according to advisers.

White House officials are discussing the need to bring in another high-level attorney, someone with the skills of former White House attorney Emmet Flood, who worked on President Bill Clinton’s impeachment team.

Earlier in the month, White House officials had said that former congressman Trey Gowdy was coming aboard to assist the legal team but later said he was unable to begin until January because of federal lobbying rules.

Meanwhile, the president has ramped up calls to lawmakers in recent days, trying to make his case individually to every Republican senator, according to a senior administration official.

Sen. Lindsey O. Graham (R-S.C.), who met with Trump at the White House on Thursday for lunch, said the president was defiant. “He just kept saying he did nothing wrong, he did nothing wrong,” Graham said in an interview. “He said it over and over.”

Graham said he wanted to see a more aggressive legal and communications team: “I was on the receiving end on the Clinton team. They were good. They knew what they were doing.”

“I hope we can get a more coordinated message,” Graham added. “They are working on it.”

Spencer S. Hsu and Tom Hamburger contributed to this report.