During the past few years, we have learned that, almost no matter how outrageous or potentially dangerous Donald Trump’s actions and words are, senior Republicans in Congress, on whose support Trump ultimately depends, won’t break with him. If anything, Trump’s grip on the Republican Party, and particularly on its process of selecting candidates, seems to be getting stronger.

It is therefore tempting to dismiss the growing protests against Trump’s decision to revoke the former C.I.A. director John Brennan’s security clearance as just another summer squall in the nation’s capital, one that will quickly blow over. But possibly—just possibly—this could turn out to be a significant political moment.

The blowback intensified on Thursday, when seven former C.I.A. directors issued a public letter supporting Brennan and denouncing the President’s decision. “We all agree that the president’s action regarding John Brennan and the threats of similar actions against other former officials has nothing to do with who should and should not hold security clearances—and everything to do with an attempt to stifle free speech,” the letter said. The letter’s signatories included William Webster, George Tenet, Porter Goss, Michael Hayden, Leon Panetta, David Petraeus, and Robert Gates, whose tenures as the head of the C.I.A. spanned five Presidents, from Ronald Reagan to Barack Obama. (Five former deputy directors of the Agency also signed the letter.)

That is quite a list. Even the stoutest Trump defenders will have difficulty describing the letter as a partisan political ambush, although, of course, that will not stop them from trying. And the letter wasn’t the only protest of its kind directed at Trump this week, or even the most cutting.

Also on Thursday, William McRaven, a retired Navy admiral who oversaw the 2011 Navy SEAL raid that killed Osama bin Laden, published an op-ed in the Washington Post, which begins, “Dear Mr. President, Former CIA director John Brennan, whose security clearance you revoked on Wednesday, is one of the finest public servants I have ever known.” The piece went on, “Therefore, I would consider it an honor if you would revoke my security clearance as well, so I can add my name to the list of men and women who have spoken up against your presidency.”

McRaven, who from 2011 to 2014 led the U.S. Special Operations Command, didn’t stop there. In a withering assessment of Trump’s Presidency, he continued, “Through your actions, you have embarrassed us in the eyes of our children, humiliated us on the world stage and, worst of all, divided us as a nation. If you think for a moment that your McCarthy-era tactics will suppress the voices of criticism, you are sadly mistaken. The criticism will continue until you become the leader we prayed you would be.”

The op-ed quickly went viral, and no wonder. Unlike Brennan, the sixty-two-year-old McRaven hasn’t recently been a fixture on social media or the cable talk shows. The son of a Second World War fighter pilot, he spent thirty-seven years in the military before retiring, in 2014, and becoming chancellor of the University of Texas system. In asking the President to revoke his security clearance, he was taking a step that could have negative financial consequences for him and his family. (Many retired military and intelligence figures parlay their security clearances into valuable consulting gigs.) And in addressing Trump personally but publicly, he was sounding a clarion call to other ex-military and intelligence officials who hitherto had kept silent.

On Thursday afternoon, Richard Haass, the president of the Council on Foreign Relations, wrote on Twitter that McRaven’s letter “could well be the closest we have come to a Joseph Welch ‘Have you left no sense of decency?’ moment that in many ways broke the McCarthy fever.” Haass was referring to the famous moment during a congressional hearing, in June, 1954, when Welch, the chief counsel to the U.S. Army, challenged Joseph McCarthy, the Republican senator from Wisconsin who for years had carried out a hysterical smear campaign against people he suspected—or claimed to suspect—of being Communists.

On Friday morning, I spoke with Haass about McRaven’s intervention and the former C.I.A. chiefs’ letter. “I think there has been something of a reluctance to take on this President—we see it among Republican politicians, business leaders, and members of the foreign-policy establishment,” Haass said. “People have been reluctant to do it because he can come back hard at you on Twitter. He can mobilize the machinery of government against you. This represents a moment when members of the élite, the establishment, are standing up and challenging him directly. McRaven created something of a precedent, almost a space, into which others can move into.”

To be sure, the former C.I.A. chiefs didn’t go as far as McRaven did. They didn’t attack Trump’s over-all record, call him an embarrassment, or ask him to revoke their security clearances in solidarity with Brennan. But, Haass noted, “They were willing to put their names to something that will not go down well in the White House. It is one thing to take on an individual like John Brennan. But I don’t think the White House counted on this type of reaction. These are patriots. Many of them have served in the military. A lot of members of the Trump base will respect these people.”

We shall see if that turns out to be true. For months now, Trump, his lawyer Rudy Giuliani, and pro-Trump media figures like Sean Hannity have been vilifying former senior government officials with long and distinguished records, including Brennan; James Clapper, the former director of National Intelligence; and James Comey, the former F.B.I. director. In the coming days and weeks, the vilification campaign may well extend to McRaven and the former intelligence chiefs who signed the public letter. Will Trump’s base care more about them than it did about Clapper?

Meanwhile, it looks like more arbitrary edicts from Trump are on the way. On Friday, the Washington Post reported that the President was “eager to move against others on the security clearance review list and could act soon.” A bit later, Trump said he intended to move “very quickly” to revoke the clearance of Bruce Ohr, a senior Justice Department official who, in 2016, had contacts with Christopher Steele, the former British intelligence officer who compiled a controversial dossier detailing alleged ties between Trump and Russia.

On Capitol Hill, some Democrats have expressed concerns that Trump’s actions are intended to silence people who might serve as witnesses in any eventual impeachment process or other legal proceeding. Most senior Republicans, predictably enough, are keeping quiet or expressing support for the President’s treatment of Brennan. The “President has full authority to revoke [Brennan’s] security clearance as head of the executive branch,” Richard Burr, the chairman of the Senate Intelligence Committee, said.

Haass, who worked in the White House under George H. W. Bush, and in the State Department under Colin Powell, conceded that, especially in the run-up to the midterms, Republicans are unlikely to come out strongly against Trump’s actions, much less hold hearings on them. But he pointed out that, back in 1954, a substantial amount of time—six months—elapsed between Welch’s exasperated remonstration of McCarthy and the Senate’s historic vote to censure the Wisconsin senator, which put an end to his reign of terror. “That statement took on more significance with the passage of time,” Haass said. “Sometimes, only in retrospect do actions or words emerge as a key moment, or a tipping point. We’ll only know in retrospect if this is one of those moments.”

A previous version of this post misstated McRaven’s role from 2011 to 2014.