On October 17, 1974, Gerald Ford took a step that some supporters of Presidential power and prerogative bemoan to this day. Wearing a blue suit, with a pin-striped shirt and a striped tie, Ford took a seat in a Capitol Hill hearing room, looked up at the members of a Democratic-controlled House Judiciary subcommittee arrayed on a dais above him, and testified for two hours. Ford was only the third President, after Abraham Lincoln and Woodrow Wilson, to voluntarily waive executive privilege and testify before Congress. A President answering questions from members of a co-equal branch of government marked, depending on one’s perspective, either a perversion of America’s constitutional order or the embodiment of it. Ford hoped that his appearance would help heal the country.

That September, just a month after Ford had assumed the Presidency, he stunned Americans by issuing a full and unconditional pardon of the former President Richard Nixon. The decision, which he announced in a surprise Sunday-morning television address, outraged many and, arguably, doomed his Presidency. Ford’s approval rating dropped from seventy-one per cent to fifty per cent.

The House Judiciary Committee, which had passed three articles of impeachment against Nixon that July, immediately launched an investigation into whether Ford had secretly promised Nixon a pardon in exchange for the Presidency. “There never was at any time any agreement whatsoever concerning a pardon to Mr. Nixon if he were to resign and I were to become President,” Ford testified. “I was absolutely convinced then as I am now that if we had had this series, an indictment, trial, conviction, and anything else that transpired after that, that the attention of the President, the Congress, and the American people would have been diverted from the problems that we have to solve.”

Forty-five years later, President Donald Trump and his allies are invoking an unprecedented form of executive privilege that would systematically undermine Congress’s power and the post-Watergate norms established by Ford. Those practices, followed by six subsequent Republican and Democratic Presidents, involve the executive branch coöperating, at least in limited form, with congressional investigations and subpoenas. Failure to do so, it was assumed, could hurt a President politically.

On Monday, the Democrat-controlled House Judiciary Committee issued a subpoena for Donald McGahn, the former White House counsel and a key figure in the Mueller report, to appear before it and present documents related to investigations of the President. On Tuesday, in a step never taken before by a modern President, Trump declared that he would bar McGahn—and all current and former Administration officials—from testifying before Congress, and, further, that the Administration would ignore all subpoenas from the legislative branch. “We’re fighting all the subpoenas,” Trump told reporters outside the White House on Wednesday. “These aren’t, like, impartial people. The Democrats are trying to win 2020.”

As Peter Baker recently noted in the Times, it has been politically unthinkable since Watergate that a President could fire an F.B.I. director who was investigating him or his associates, force out an Attorney General for failing to protect him from an investigation, or offer pardons to potential witnesses against him. Now Trump is flatly dismissing Congress’s authority to conduct oversight of the executive branch. Unless Trump is stopped by court rulings, his defiant brand of politics and his sweeping control of the Republican Party will redefine the balance of power between the President and Congress for decades.

Trump’s approach is the opposite of the one that Ford adopted. At first, Ford’s decision to testify appeared to pay off—the Judiciary Committee soon closed its investigation. But the reprieve for the Presidency was short-lived. That November, Democrats won a landslide victory in the midterm elections, gaining a two-thirds majority in the House and sixty seats in the Senate. With Watergate and Vietnam fuelling public suspicion of Presidential power, Congress reached the apex of its power in modern American politics. Already, in 1973, overriding a veto by Nixon, Congress had enacted the War Powers Resolution, which required Presidents to halt military operations after sixty days if they were not approved by Congress. In 1974, it passed amendments that strengthened the Freedom of Information Act, giving the public greater access to government documents. Antonin Scalia, then a Justice Department official, argued to Ford that the amendments infringed on the secrecy of the F.B.I. and other intelligence agencies. Ford vetoed the amendments, newspaper editorials across the country assailed him, and Congress easily overrode his veto, with a 65–27 vote in the Senate and a 371–31 vote in the House. The legislative tide continued under President Jimmy Carter. The Ethics in Government Act, which, among other things, created the Office of Special Counsel and required officials to file financial-disclosure forms, was enacted in 1978. The same year, the Foreign Intelligence Surveillance Act was signed into law, limiting the ability of the executive branch to surveil Americans without a court order.

Ford, in an effort to regain public trust, also voluntarily enacted multiple reforms. After a congressional investigation uncovered decades of improper activities by both the C.I.A. and the F.B.I., he issued an executive order barring political assassinations and supported the establishment of congressional committees to oversee the intelligence agencies.

Yet Ford also fought back against what he viewed as an erosion of Presidential power. In his two and a half years as President, he issued sixty-six vetoes, the highest rate of any modern President. Democrats overrode them a dozen times. Donald Rumsfeld, a former congressman from Illinois who, in 1974, at the age of forty-two, was named Ford’s chief of staff, fiercely defended executive-branch power. So did Rumsfeld’s deputy, Dick Cheney, who, a year later, at the age of thirty-four, became chief of staff, when Ford named Rumsfeld as his Secretary of Defense.

Subsequent Republican and Democratic Presidents have also fought congressional oversight. The Reagan and Bush Administrations sparred with the legislative branch throughout the Iran-Contra scandal. Bill Clinton battled House investigations into the Whitewater land deal and the White House travel office, and also into his impeachment, for perjury and obstruction of justice, during the Monica Lewinsky scandal. During George W. Bush’s Administration, Vice-President Dick Cheney contended that the executive branch had the power to secretly surveil Americans without obtaining a court order from the FISA court. (After the Times revealed the surveillance, the Administration altered the program and obtained judicial and congressional approval for it.) Barack Obama, too, resisted congressional investigation. His Attorney General Eric Holder was held in contempt of Congress after he declined to provide a Republican-controlled House committee with documents it requested regarding the so-called Fast and Furious investigation. The Administration, along with Democrats in Congress, argued that the investigation, which looked into the sale of illegal guns as part of a counter-narcotics operation, was a meritless effort to score political points. After a six-year legal battle, the case was settled by the Trump Administration, which turned over a limited number of documents to Congress.

The idea that a President such as Ford would testify before Congress and somehow set the record straight now seems quaint. Investigations are increasingly dismissed by voters as fishing expeditions. The eyes of liberals glaze over when Benghazi is mentioned. (That probe succeeded, at least politically, by uncovering Clinton’s use of a private e-mail server.) The eyes of conservatives glaze over when Trump-Russia is cited. (That inquiry succeeded, at least politically, by uncovering evidence of attempted obstruction of justice by Trump.) Stonewalling investigations and rallying one’s political base appear to be a savvy way to secure a high voter turnout, which is needed to win elections in an increasingly divided country. In this case, Trump is exacerbating a preëxisting trend in American politics, not creating one. The question is whether a 2020 candidate can emerge who can restore lost trust and norms and garner votes.