Friday’s arrest of Roger Stone, the longtime political adviser to Donald Trump, was greeted with a striking signal from the man who will play a critical role in deciding whether this scandal-plagued president will face impeachment.

“Roger Stone, Paul Manafort, Michael Cohen, Rick Gates, Michael Flynn,” tweeted House Judiciary Committee chair Jerry Nadler, who ticked off the names of presidential aides and associates who have since the 2016 election been charged, indicted, pleaded guilty, or been convicted of criminal wrongdoing. Then, reaching back into the playbook from a previous impeachment inquiry, he asked the classic question: “What did the President know and when did he know it?”

That’s a reference to a question asked in 1973 by Tennessee Senator Howard Baker, the ranking Republican member of the Senate Select Committee on Presidential Campaign Activities. The panel was better known as the Senate Watergate Committee, and Baker had been seen as a key defender of Richard Nixon against what the Republican president decried as—and this will sound familiar—a “witch hunt.” In June of 1973, when White House counsel John Dean appeared before the committee, Baker began his round of questioning with a statement of intent.

“My primary thesis is still, what did the president know, and when did he know it?” the senator explained. Baker was seeking clarity, a clarity that many Republicans thought would protect Nixon by showing that he did not know what his associates were doing. But, as The Christian Science Monitor recalled, “Dean turned this question around. Among other things, he charged that Nixon had been involved in discussions about clemency for those who had carried out and organized the break-ins, as well as talks about payoffs. Dean said the president had continued these activities even after he, as White House counsel, had warned his boss of a ‘cancer’ on the presidency.”

White House tapes and testimony from other Nixon associates would quickly confirm that Dean was telling the truth. Suddenly, Baker’s question sounded like a challenge to Nixon. And a damning one at that. The details of what the president knew and when the president knew it would form the underpinnings for articles of impeachment that would be approved in the summer of 1974 with bipartisan support from the House Judiciary Committee. Threatened with accountability, Nixon cut and ran.

Nadler’s recollection of the essential question of the Watergate era was a knowing response to the news that Stone had been arrested on the basis of a seven-count indictment from a criminal case brought by special counsel Robert Mueller. According to The New York Times, the Stone indictment “revealed on Friday the most direct link yet between the Trump campaign’s and WikiLeaks’ parallel efforts to use Democratic Party material stolen by Russians to damage the election campaign of Hillary R. Clinton.”

Stone identifies as a Trump ally and says he will not turn on the president. Trump tweeted last month that Stone has “guts!;”—perhaps encouraging his longtime fixer and friend to remain loyal, and quiet, even as other Trump aides and employees were cooperating with investigators and being talked up as potential witnesses before the Judiciary Committee.

Nadler’s tweet served as a reminder that this is not the first time that Stone has been associated with a scandal-plagued commander in chief who faces scrutiny from prosecutors and members of Congress. Stone started his political journey doing dirty tricks for Nixon’s 1972 presidential campaign and then took a low-level job in the administration.

So Nadler’s mention of Baker’s line was ironic.

But this mention of the “what did the President know and when did he know it?” quote also went to the heart of the matter that Nadler and other members of Congress will have to deal with as the Mueller inquiry unravels the details of wrongdoing by the president’s associates—and, potentially, the president. Nadler has been cautious in his approach to impeachment questions, just as House Judiciary Committee chair Peter Rodino was during Watergate. Nadler will not rush to judgment. But the current Judiciary Committee chairman will not rush away from it.

Nadler has employed a series of questions to establish a standard for consideration of impeachment: “One, were impeachable offenses committed, how many, et cetera. Secondly, how important were they? Do they rise to the gravity where you should undertake an impeachment?” Now, he has raised the most historically and practically consequential question: “What did the President know and when did he know it?”

Advocates for impeachment, such as this writer, believe that the president has already crossed the line. Some Democratic members of Congress agree. Most, including Nadler, have signaled that they are waiting to see what the Mueller inquiry produces. But that does not mean that we lack measures of progress toward the accountability moment that, to the view of a growing number of Americans, seems inevitable.

Back in 2017, when Trump fired then–FBI Director James Comey, Congressional Progressive Caucus co-chair Mark Pocan, said: “Trump firing Comey reminds me of the doomsday clock, but maybe we should start an impeachment one.” The firing of Comey, Pocan suggested, had “moved us an hour closer to midnight.”

As the discussion turns to what Donald Trump knew, and when he knew it, it is safe to say that the impeachment clock is ticking faster.