WASHINGTON — Two days after the 2020 election that Donald J. Trump refused to admit he lost, his oldest son, Donald Trump Jr., made an urgent recommendation: “Fire Wray.”

The younger Mr. Trump did not explain in the text he sent why it was necessary to oust Christopher A. Wray, the F.B.I. director his father himself had appointed more than three years earlier. He did not have to. Everyone understood. Mr. Wray, in the view of the Trump family and its followers, was not personally loyal enough to the departing president.

Throughout his four years in the White House, Mr. Trump tried to turn the nation’s law enforcement apparatus into an instrument of political power to carry out his wishes. Now as the F.B.I. under Mr. Wray has executed an unprecedented search warrant at the former president’s Florida home, Mr. Trump is accusing the nation’s justice system of being exactly what he tried to turn it into: a political weapon for a president, just not for him.

There is, in fact, no evidence that President Biden has had any role in the investigation. Mr. Biden has not publicly demanded that the Justice Department lock up Mr. Trump the way Mr. Trump publicly demanded that the Justice Department lock up Mr. Biden and other Democrats. Nor has anyone knowledgeably contradicted the White House statement that it was not even informed about the search at Mar-a-Lago beforehand, much less involved in ordering it. But Mr. Trump has a long history of accusing adversaries of doing what he himself does or would do in the same situation.

His efforts to politicize the law enforcement system have now become his shield to try to deflect accusations of wrongdoing. Just as he asserted on Monday that the F.B.I. search was political persecution, he made the same claim on Wednesday about the New York attorney general’s unrelated investigation of his business practices as he invoked his Fifth Amendment right to avoid testifying because his answers could incriminate him.

“Now to flip the script and falsely claim that he’s the victim of the exact same tactics that he once deployed is just the rankest hypocrisy,” said Norman L. Eisen, who served as special counsel to the House Judiciary Committee during the first Trump impeachment. “But consistency, logic, evidence, truth — those are always the first to go by the board when a democracy comes under assault from within.”

Mr. Trump’s Republican allies argue that he was not the one who undercut the apolitical tradition of the F.B.I. and law enforcement, or at least he was not the first to do so. Instead, they maintain, the system was corrupted by the bureau’s leadership and even members of the Obama administration when Mr. Trump and his campaign were investigated for possible collusion with Russia during the 2016 campaign, an inquiry that ended with no charges of conspiracy with Moscow.

The former president’s camp has long pointed to text messages between a pair of F.B.I. officials that sharply criticized Mr. Trump during that campaign and to surveillance warrants obtained against an adviser to Mr. Trump that were later deemed unjustified. The Justice Department acknowledged the warrants were flawed, and an inspector general faulted the F.B.I. officials for their texts. But the inspector general found nothing to conclude that anyone had tried to harm Mr. Trump out of political bias.

In a letter to Mr. Wray on Wednesday, Senator Marco Rubio of Florida, the top Republican on the Senate Intelligence Committee, alluded to the history of the F.B.I.’s previous investigation of Mr. Trump to cast doubt on the current inquiry that led to Monday’s search for classified documents that the former president may have improperly taken when he left office.

“The F.B.I.’s actions, less than three months from the upcoming elections, are doing more to erode public trust in our government institutions, the electoral process and the rule of law in the U.S. than the Russian Federation or any other foreign adversary,” Mr. Rubio said in the letter.

The search was approved by a magistrate judge and high-level law enforcement officials required to meet a high level of proof of possible crimes. Attorney General Merrick B. Garland, himself a former appeals court judge who was appointed by Mr. Biden with bipartisan support and whose caution in pursuing the former president until now had generated criticism from liberals, has offered no public explanation so far.

The degree to which Mr. Trump has succeeded in promoting his view of a politicized law enforcement system was evident in the hours after the F.B.I. search on Monday when many Republicans, including Representative Kevin McCarthy of California, the House minority leader, wasted little time assailing the bureau’s action as partisan without waiting to find out what it was based on or what it turned up.

Even Republicans who have been critical of the former president in the past felt compelled to challenge the validity of the search. Senator Mitch McConnell of Kentucky, the minority leader who excoriated Mr. Trump for his role in the Jan. 6 attack on Congress, waited 24 hours but finally spoke out on Tuesday to question whether something untoward had happened.

“The country deserves a thorough and immediate explanation of what led to the events of Monday,” he said in a statement. “Attorney General Garland and the Department of Justice should already have provided answers to the American people and must do so immediately.”

But some law enforcement veterans said Mr. Trump simply projects his own views onto others. “Trump may actually believe that Merrick Garland is serving a political agenda because he has trouble processing anything else,” said Michael R. Bromwich, a former Justice Department inspector general. “Trump simply doesn’t understand people like Garland and the top leadership of D.O.J. and the F.B.I. because their values are so alien to him.”

The F.B.I. has a history at the intersection of politics and investigations. Under J. Edgar Hoover, its longtime director, the bureau bugged and pursued domestic opponents of the federal government, at times serving as a political tool of various presidents of both parties. But with revelations of past abuses after Hoover’s death in 1972, Congress and the F.B.I. sought to cast off the bureau’s history and transform it into a more professional, politically neutral organization.

F.B.I. directors were appointed to 10-year terms to make them less subject to presidential whims, a new office of professional responsibility was established, the House and the Senate set up intelligence oversight committees, and other reforms were enacted to remove the bureau from politics. Along the way, the bureau earned the respect of both parties and many Americans in the last half-century.

That built-up store of public credibility has eroded significantly in the Trump years. The proportion of Americans who told Gallup pollsters that they thought the F.B.I. was doing a good job fell from 57 percent in 2019 to 44 percent in 2021.

And while public approval of the bureau had long been bipartisan, views have now diverged along party lines. In Mr. Trump’s first year in office, as he attacked the F.B.I. over the Russia investigation, the share of Republicans who had a favorable view of the bureau fell to 49 percent from 65 percent in surveys by the Pew Research Center while remaining steady among Democrats at 77 percent.

“Trump upset the post-1970s status quo when he became president, tipping off-balance over 40 years of an imperfect-though-laudable D.O.J.- and F.B.I.-constructed culture of apolitical independence,” said Douglas M. Charles, a historian of the F.B.I. at Penn State and the author or editor of several books on the bureau. “It seems to me Trump has really put that culture and the F.B.I. itself to the test to expose the weaknesses and limitations of the post-1970s system.”

Mr. Trump’s view of the law enforcement system has been shaped by his own encounters with it, starting as a young developer in New York when the Justice Department sued his family company in 1973, accusing it of racial discrimination. Eventually, the Trump firm settled and agreed to change its policies, leaving a bitter taste in Mr. Trump’s mouth.

By the time he ran for office, Mr. Trump viewed the justice system through a political lens. He led rally crowds in “lock her up” chants as he suggested he would imprison his opponent, former Secretary of State Hillary Clinton, who was investigated but not prosecuted for improper handling of classified information — much as he is now suspected of doing.

After winning, Mr. Trump saw law enforcement agencies as another institution to bend to his will, firing the F.B.I. director James B. Comey when he declined to pledge personal loyalty to the president or publicly declare that Mr. Trump was not a target of the Russia inquiry. The president later fired Attorney General Jeff Sessions for recusing himself from that investigation and therefore not protecting Mr. Trump from it.

During his time in office, Mr. Trump repeatedly called on the Justice Department and the F.B.I. to investigate his foes and let off his friends. He publicly criticized the prosecutions of campaign advisers like Paul J. Manafort and Roger J. Stone Jr., eventually reversing their convictions with pardons after they refused to testify against him. He complained when two Republican congressmen were charged shortly before the 2018 midterm elections because it could cost the party seats.

Frustrated with Mr. Wray, Mr. Trump sought to install a more supportive director at the F.B.I. in 2020, backing down after protests by Attorney General William P. Barr. By that fall, as the president trailed in the polls for re-election, he pushed for the prosecution of Mr. Biden’s son Hunter and lashed out at Mr. Barr and Mr. Wray for not prosecuting Democrats like the elder Mr. Biden and Barack Obama because of the Russia inquiry.

“These people should be indicted,” Mr. Trump said. “This was the greatest political crime in the history of our country, and that includes Obama and it includes Biden.”

After losing his bid for a second term, Mr. Trump ultimately disregarded his son’s advice and did not fire Mr. Wray, but in his final weeks in office pushed the Justice Department to help him overturn the election. Mr. Barr rebuffed Mr. Trump and publicly rejected the false election claims before resigning.

Mr. Trump repeatedly pressed Mr. Barr’s successor, Jeffrey A. Rosen, to go along with his scheme to discredit the election results and came close to firing him when he would not and installing an ally who would, Jeffrey Clark. The president was blocked only when told that every senior Justice Department official would resign in protest.

That was his last chance to influence law enforcement from the inside, at least for now. So from the outside, he rails against what he calls the injustice of a law enforcement agency run by his own appointee.