"I would say, 'You are obviously a person of integrity and you take it seriously when you hear about wrongdoing,'" Brockovich, the environmental and consumer activist, said in a recent telephone interview. "The name 'whistleblower' gets a bad connotation, and I've never understood that."
Labeled #FakeWhistleBlower by Trump, the official remains unknown to the public but has already been linked to such whistleblowers of the past as Brockovich, Daniel Ellsberg and Edward Snowden.
Presidents, military leaders and corporate executives have raged against them, but whistleblowers have been around as long as the country itself.
The world's first whistleblower protection legislation is widely considered to have been passed in the U.S., in 1778, and whistleblowers have since helped advance or break scandals ranging from Enron to lies about the Vietnam War. Whistleblowers have inspired movies ("Erin Brockovich," ''Silkwood," ''Serpico") and countless books, most recently Snowden's best-selling memoir, "Permanent Record."
"Whistleblowing is as American as apple pie," says author Allison Stanger, whose "Whistleblowers: Honesty in America from Washington to Trump" came out last week.
Whistleblowers have a long, diverse and complicated history, right down to the definition and spelling of the word. It dates back at least to the 19th century, when a police officer trying to warn citizens about a riot was referred to as a "whistle blower."
In the 20th century, "whistle blowers" became "whistle-blowers" and were at times associated with sports referees or with "snitches" or "rats" who violated a code of silence.
The expression was rebranded in the 1970s by consumer advocate Ralph Nader and moved closer to its current understanding of someone who calls out corporate or government wrongdoing. "Whistleblower" is now one word, generally unhyphenated, and defined by activists as someone who exposes wrongdoing, often from the inside at personal risk.
"It isn't surprising to me that the Trump-Ukraine whistleblower is from the intelligence community," says Danielle Brian, executive director of the watchdog Project on Government Oversight. "Whistleblowers don't plan to become whistleblowers. People think of whistleblowers as some kind of anarchist, but they tend to be more conservative than we realize and to see the world in terms of right and wrong."
Whistleblowing may begin with a moment of conscience, but there is no single path to action, no single kind of crime exposed or agreed-upon canon of whistleblowers. The Trump-Ukraine official worked within established government channels, but others have openly broken laws.
Ellsberg has said he expected to spend the rest of his life in prison after leaking the Pentagon Papers, a Defense Department study of the government's involvement in Vietnam that he helped prepare (Charges against him of violating the Espionage Act were thrown out after it was learned that the Nixon administration had burglarized his psychiatrist's office.)
Snowden, a former intelligence contractor who now lives in Russia, leaked files on massive government surveillance and would almost surely face prosecution if he returned to the U.S.
Whistleblowers have revealed financial fraud at Enron, excessive costs of military weapons systems, and the tampering with tobacco at Brown & Williamson, a scandal from the 1990s that inspired the movie "The Insider."
Louis Clark, who heads the nonprofit Government Accountability Project, cites Robert MacLean, the federal air marshal who in 2003, just two years after the Sept. 11 terrorist attacks, disclosed government plans to cut costs by removing air marshals from all overnight long-distance flights.
"Very soon after he went public, they reversed that decision," Clark says.
Experts on whistleblowing agree on the importance of whistleblowers, but not on who deserves to be called one.
Snowden was criticized not just by government leaders but by the liberal historian Sean Wilentz, who has noted that Snowden didn't just unearth information on domestic surveillance, but on foreign programs such as the tracking of the Taliban.
"Regardless of whether any of these documents in any way compromised U.S. interests abroad, they were plainly not the revelations of 'whistle-blowers' seeking to secure Americans' constitutional rights," Wilentz wrote in The New Republic in 2014. "They are the revelations of leakers, out to damage their bugaboo national security behemoth."
Clark says that Mark Felt, the once-anonymous "Deep Throat" of Watergate, was a whistleblower who made a critical contribution to the scandal that led to Nixon's resignation. Former Nixon White House counselor John W. Dean, whose own whistleblower status has been debated since his devastating testimony during the 1973 Watergate hearings, says Felt was more a disgruntled bureaucrat than advocate for justice.
"He had an agenda," Dean says of Felt, an associate director of the FBI at the time of Watergate. "He wanted to be director of the FBI and thought if he could remove (acting director) L. Patrick Gray, Nixon would pick a (J. Edgar) Hooverite like himself."