Washington (CNN) Retired CIA analyst Gail Helt has lived her life largely in the shadows. But after President Donald Trump revoked former CIA Director John Brennan's security clearance, Helt joined almost 200 colleagues to sign an unprecedented public letter of protest.

"This is very rare," Helt told CNN of the letter, signed by more than 175 former intelligence community professionals. "I can't remember another instance in which such a diverse group of people came together like this to essentially say 'this is wrong' to a president."

The growing momentum behind the protest by former career intelligence and national security professionals concerned about Trump's action is remarkable, particularly for nonpolitical appointees and employees whose names are nowhere on social media, in news articles or in books.

"I've never seen anything like it," said Larry Pfeiffer, former chief of staff to Gen. Michael Hayden, the former CIA and National Security Agency director. Referring to high-profile Republican national security experts who signed letters opposing Trump's nomination, Pfeiffer said that "even the 'Never Trump' letters had fewer signatories and nowhere near the number of career" intelligence people.

Rank and file

From high-ranking former directors of the CIA and the Office of National Intelligence to CIA analysts and linguists who may never have testified publicly before Congress, or even posted to Facebook, intelligence officials have made the choice to sign the letter stressing their concern that a political litmus test is being applied to a field in which national security is supposed to be held above partisan concerns.

After former CIA Director George Tenet convened a group of former directors over email to decide what to do about Trump's punitive response to Brennan, their decision to go public led to two letters over the course of the weekend. Responses are still pouring in, according to the Tenet letter's organizers.

The majority of those who signed the second letter "are rank and file career intelligence officers, national security professionals who would never imagine speaking out against the president," said Nick Shapiro, former CIA deputy chief of staff and the organizer of one of the letters.

Signatories from all corners of the national security community spoke up, Shapiro said. "They are saying this is not what I risked my life for," to have the President utilize a tool of national security to "punish someone for speaking their mind," he said.

Helt, who now teaches about the world of intelligence at a small school in Tennessee, is one of many intelligence professionals who once lived under the radar but have taken a more public stance during the Trump presidency.

Helt has occasionally used Twitter to voice her views on intelligence matters, including her opposition to the nomination of CIA Director Gina Haspel because of her supervision of a black site in Thailand where harsh interrogations were conducted.

Some former senior officials have been speaking up for the community for years, including Hayden, who wrote a book about the business of intelligence called "Playing to the Edge," in part fighting back against criticism the community endured after then-NSA contractor Edward Snowden exposed a top-secret surveillance program that collected telephone metadata. That's gotten him into a fight or two with privacy and civil liberties activists.

Now he's joined his colleagues in speaking out in opposition to the President.

JUST WATCHED WH defends decision to revoke clearances Replay More Videos ... MUST WATCH WH defends decision to revoke clearances 01:34

Not everyone agrees the letter was the right move. Some believe Brennan's vocal, personal opposition to the President has "rankled" some who may disagree with him, weakening the strength of the letter.

While the letter clarifies that the signatories "do not necessarily mean that we concur with the opinions expressed by former CIA Director Brennan or the way in which he expressed them," some like former CIA officer Daniel Hoffman, a Fox News contributor, worry the strategy won't be effective.

"President Trump picked a controversial figure" to revoke a security clearance, he said. "Some people like him, but he's been so extreme, so partisan, so beyond what I think we would ever expect from a retired senior official."

Hoffman worries that the politics, not unlike retired Lt. Gen. Michael Flynn's "lock her up" chants about Hillary Clinton during the election, could be inappropriate and fuel adversaries' attempts at sowing discord in American society.

Shapiro disagreed, suggesting the letter was clear and that Brennan's character is a key reason so many former officials clamored to sign on.

Acts of quiet solidarity

"Whether people agree with Brennan or not, everyone seems to agree he has a right to speak his opinion," Shapiro said. "But it is also true that John Brennan's integrity, his credibility and his professionalism are important factors. If John Brennan was how the White House was trying to portray him ... then you would not have seen 177 former officials rallying together behind him on Sunday."

The letter and its supporters haven't gone unnoticed within the intelligence community. While former officials in touch with current officers said there's little high-level discussion of the matter -- an indication "people were putting their heads down ... no doubt hoping all this would just go away," one said -- multiple former intelligence officials told CNN they've received quiet notes of thanks, for speaking up and putting their names to the pledge when others can't.

And, according to one former intelligence official in touch with current officials, there are small, quiet acts of solidarity and protest within the CIA.

At the agency, a wall usually lined with former directors' portraits is being updated and switched out. As a temporary placeholder, a book about former CIA directors has been placed by the wall. Over the weekend, someone at the agency opened the book to the chapter about Brennan and left it there.