Matthew Whitaker’s appointment to be acting attorney general is not only legal, but it’s also similar to moves made by the Obama and Bush administrations, the Justice Department’s office of legal counsel said Wednesday, mounting a firm defense of President Trump’s pick.

The office, which serves as the government’s chief internal legal voice, said it found more than 160 such appointments dating back to the founding era that involved someone who hadn’t been confirmed by the Senate performing the duties of a high official, such as secretary, at the State, Treasury, War or Interior departments.

The office of legal counsel’s 20-page legal opinion provides needed firepower to a White House battling conservative and liberal legal scholars who have said Mr. Trump is violating the Constitution by installing Mr. Whitaker, who had been chief of staff at the department, over Senate-confirmed officials such as Deputy Attorney General Rod Rosenstein.

A senior Justice Department official said Mr. Trump sought guidance before his decision last week to oust Attorney General Jeff Sessions, and the office of legal counsel assured him that he was on firm ground in flexing a 1998 law to elevate Mr. Whitaker.

“What we’re talking about here is the constitutional question. The question is: Can Congress provide the president with this authority?” the official told reporters. “The Vacancies Reform Act unequivocally does provide the president with that authority.”

Mr. Whitaker’s designation has become a major flashpoint in the week since it happened.

Congressional Democrats, who fear Mr. Whitaker will curtail the special counsel’s investigation into the 2016 election, Russian meddling and Trump campaign figures’ behavior, have vowed investigations and demanded recusals.

Maryland Attorney General Brian Frosh, a Democrat, has filed a motion in an ongoing case asking a judge to invalidate Mr. Whitaker’s designation and to proactively install Mr. Rosenstein as the acting attorney general, blocking Mr. Trump from making a pick.

Mr. Frosh and other Trump opponents say that laws governing the Justice Department list the deputy attorney general as the natural successor if the attorney general is no longer in office.

But the office of legal counsel opinion says those laws aren’t exclusive.

The office said it has opined as far back as 2007, well before Mr. Trump was president, that the Vacancies Reform Act could also be used to designate an acting chief. The office of legal counsel concluded at that time, and maintains today, that the president can trigger the act.

Trump critics say a bigger question than the law is the Constitution. They point to a clause that suggests principal officers must be confirmed by the Senate.

“We cannot tolerate such an evasion of the Constitution’s very explicit, textually precise design,” Neal K. Katyal, a former high-ranking Obama administration litigator, and George T. Conway III, husband to White House adviser Kellyanne Conway, wrote in a New York Times op-ed last week.

“Constitutionally, Matthew Whitaker is a nobody. His job as Mr. Sessions’ chief of staff did not require Senate confirmation. … For the president to install Mr. Whitaker as our chief law enforcement officer is to betray the entire structure of our charter document,” the two lawyers wrote.

The office of legal counsel, though, found more than 160 instances before the Civil War of officials who didn’t have Senate confirmation serving as secretaries of state, Treasury, war and interior. They also found an assistant attorney general, who was not Senate-confirmed, serving as acting attorney general in 1866.

“Mr. Whitaker’s designation is no more constitutionally problematic than countless similar presidential orders dating back over 200 years,” wrote Steven A. Engle, the assistant attorney general who signed the office of legal counsel opinion.

He also pointed to three instances during the Obama administration when the president elevated a chief of staff at a federal agency to be acting head over another Senate-confirmed official.

Mr. Obama also named non-Senate-confirmed people in 2009 to serve as acting secretary at the Labor Department and acting administrator at the Small Business Administration, the office of legal counsel said.

President George W. Bush named someone without Senate confirmation to serve as acting director of the White House Office of Management and Budget, the opinion says.

None of those actions sparked the debate Mr. Whitaker’s designation has. Nor did they implicate an ongoing investigation that has already resulted in guilty pleas from some former Trump campaign figures.

The senior Justice Department official briefing reporters declined to speak about whether there were any limitations on Mr. Whitaker’s powers as acting attorney general, such as whether he can claim oversight of special counsel Robert Mueller’s investigation.

Mr. Trump’s political opponents dismissed the office of legal counsel’s opinion and predicted that the judge in Mr. Frosh’s lawsuit will settle matters.

But beyond questions of legality, they said Mr. Whitaker is a bad choice.

“He is wholly unqualified for this position, having made past statements contradicting American principles of judicial independence and having prejudged the outcome of the special counsel’s investigation he is now tasked with overseeing,” said Rep. Steny H. Hoyer of Maryland, the second-ranking Democrat in the House.

Democrats point to comments Mr. Whitaker made before he was at the Justice Department suggesting the Mueller investigation could be curtailed from within the Justice Department by limiting its budget.

Democrats, who have said Mr. Trump’s election is illegitimate, are hoping the investigation snares the president himself — though there is no concrete evidence that he is personally a target.

Mr. Trump has called the investigation a “witch hunt” and repeatedly signaled his displeasure with it.

But Senate Majority Leader Mitch McConnell, Kentucky Republican, said Wednesday he hasn’t seen evidence that the president will try to scuttle the investigation before it is complete.

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