President Nixon’s resignation from office in August 43 years ago to avoid certain impeachment should stand as a stark warning to President Trump about the consequences of trying block the investigation headed by Special Counsel Robert Mueller into Russiagate.

Last May, Trump fired FBI Director James Comey for not dropping his investigation into Trump’s former national-security adviser Michael Flynn. This action was the first major indication of the president’s deep hostility to the investigation into his campaign’s possible collusion with Russia over the 2016 presidential election. It also showed how far he was willing to go to stop it. Firing Comey actually made Trump’s situation worse. It led directly to Mueller’s appointment as special counsel, and to an investigation into whether Trump obstructed justice by firing Comey.

In July, Trump took another step by relentlessly attacking his attorney general, Jeff Sessions, for removing—recusing—himself from the Russia investigation. Trump saw the recusal as the original sin, because, without it, Mueller, one of the country’s most respected law-enforcement figures, would never have been appointed special counsel. These attacks clearly appeared designed to force Sessions to resign, allowing Trump to replace him with an AG who would not have to recuse himself and could and would fire Mueller. (In Watergate, both the AG and deputy AG resigned rather than fire the special prosecutor investigating the president.)

Trump’s plan came to naught, primarily because Sessions’s former colleagues in the Senate rebelled: The Senate Judiciary Committee Chair said he would hold no confirmation hearings on a new AG, and the Senate adjournment procedures prevented a recess appointment that doesn’t need confirmation.

One of Trump’s objectives in getting a new AG was to avoid the huge political cost of personally firing Mueller. In Watergate, when President Nixon ordered the firing of the Watergate special prosecutor because his investigation was closing in on him, it triggered Nixon’s downfall. The public outrage set in motion impeachment proceedings that resulted in Nixon’s resignation from office.

But, whatever tricks Trump may use to remove Mueller, they will not automatically shield him from accountability. His firing the Watergate special prosecutor was a ground for the articles of impeachment against Nixon voted by the House Judiciary Committee. Removing Mueller without cause at Trump’s instigation could similarly form a basis for his impeachment—a grave abuse of power intended to obstruct the investigation for the president’s personal benefit.

Trump also appears to have taken another page out of Nixon’s playbook by talking about pardons. Part of the Watergate cover-up was keeping the burglars quiet about higher-ups. To do that, Nixon authorized offering them presidential pardons—which became another ground for the impeachment vote against him. This too was a serious misuse of presidential powers.

Just before his son, son-in-law,and former campaign manager were to appear before Congress, Trump tweeted that his pardon powers were “complete.” Could the tweet have been a signal to these prospective witnesses—and others—to stonewall or not tell the truth? Could it have been an attempt to discourage Mueller from prosecuting Trump or those close to him, because pardons would just nullify any prosecutions? If so, Trump’s actions could be additional grounds for impeachment, and even for prosecution on obstruction of justice grounds.

Trump has tried to interfere with the investigation in other ways. The White House has raised flimsy and spurious conflict-of-interest claims against Mueller and his staff. Trump has also announced that investigating his finances would cross a “red line.” Others, including a close Trump ally, have said that Trump has ranted privately about firing Mueller, something Trump has recently denied. Trump has also called the idea that Russia interfered in the presidential election a hoax, fake news, and the like, even though all US intelligence agencies agree that Russia did interfere—an extraordinary and unprecedented trashing of the US intelligence agencies by a president. Although Trump is obviously trying to prove he won the election legitimately, an equally or more important motivation is to use the power of his presidency to undermine the legitimacy of the investigation and mobilize his base against it.

Another Nixon gambit taken up by Trump is trying to “stroke” the special counsel. Media reports suggest that Trump has passed the word to Mueller that he respects his investigation and won’t fire him. During Watergate, while claiming that the break-in was just a third-rate burglary, Nixon frequently met with the Justice Department prosecutor, buttering him up and beguiling him into disclosing grand-jury secrets, in order to aid the cover-up. Mueller would not likely fall for such tactics, but Trump’s attempt to use them could be both criminal and impeachable.

Despite everything, Mueller seems to be forging single-mindedly ahead, staffing up with top-notch prosecutors. Recently, the FBI conducted a pre-dawn raid on the home of Paul Manafort, Trump’s former campaign manager. Trump, however, still takes the opportunity to ping the investigation, saying he was “very surprised” and that the raid was “tough stuff.”

The clash we are witnessing is not just between Trump and Mueller or Trump and Sessions, but between Trump and democracy itself. If a president may with impunity stymie an investigation of his family, his close associates, and himself—whether by removing a respected prosecutor, suggesting the possibility of pardons to bolster false testimony, using the power of his office to attack the need for the investigation, or attacking the integrity of the prosecution—then our country has lost its way and the rule of law in in serious jeopardy.

In Watergate, another example of a spectacular presidential assault on our democratic institutions, the courts and the Congress stood up to counter the president.

Today the courts have been mostly willing to act as a check on Trump, but what can be said about Congress? Some argue that impeachment is impossible as long as Republicans control the House and Senate. It may be hard, but it is not impossible. During Watergate, even though Democrats controlled the House (where impeachments must start) and the president was a Republican, the House leadership refused to act on impeachment, despite all the derogatory information that had surfaced about Nixon’s role in Watergate, until the special prosecutor was fired. That is when the American people, angry at Nixon’s putting himself about the law, forced Congress to act.

Impeachment will not happen without hard evidence of serious abuses of power and without broad public support. If Mueller is fired or other criminally damning information emerges about Trump, then even a Republican-controlled Congress may decide to uphold the rule of law—or be compelled to do so. In 1974, the American people turned on Nixon, despite his landslide victory eleven months earlier. They determined that our democracy was more important than any president and demanded Congress hold him accountable. They can do it again.