Florida Sen. Marco Rubio makes it clear where he stands on Edward Snowden's exposure of the National Security Agency's spying programs: The situation couldn't be more dire.
"The single most damaging revelation of American secrets in our history," Rubio said when asked about the matter after a foreign policy speech at the University of Texas on April 15.
"I can say to you unequivocally that there are Americans whose lives are at risk because of those disclosures," Rubio said.
There's little question that the revelations are nearly unprecedented in the history of American espionage.
Snowden, a former CIA employee, gave a small group of reporters thousands of classified documents he found with his security clearance as a contractor for the NSA. The information detailed surveillance programs and data mining operations against world leaders and American and European citizens. Facing espionage charges, he is now living in an undisclosed location in Russia.
We can't fact-check Rubio's opinion, but we did survey a raft of historians and experts to see whether they felt the senator was making a reasonable point.
The verdict: Reasonable, perhaps, but not definitive.
The most serious instances come down to someone being killed, whether they be soldiers, covert agents or informants, said Daveed Gartenstein-Ross, a senior fellow at the Foundation for Defense of Democracies. Long-term revelations that compromise U.S. operations also can cause major damage, he said.
He compared the Snowden case with the Pentagon Papers, in which former government military analyst Daniel Ellsberg released a classified history of U.S. involvement in Indochina from World War II to 1968. The report, which the New York Times published details of in 1971, revealed the United States had secretly escalated the Vietnam War without the public's knowledge.
That's a different type of revelation, though, than spies who decided to go work for the other side. Bruce Thompson, a lecturer on history and literature at the University of California at Santa Cruz, pointed to several of what he termed "major spy-traitors of the second half of the 20th century" as revealing secrets that were more damaging than Snowden's.
Aldrich Ames, a double agent in the CIA, exposed virtually all Soviet agents of the CIA and other American and foreign services he knew of between 1985 and 1994, leading to some of those people's deaths.
There was also Robert Hanssen, an FBI analyst who sold secrets about espionage, communications, FBI moles and more to Soviet and later Russian officials between 1979 and 2001. Hanssen's case led to the production of the 2007 film Breach, which carried the tagline, "Inspired by the true story of the greatest security breach in U.S. history."
In a similar vein, the case of Julius and Ethel Rosenberg centered on a major secret being relayed to the Soviets. Ethel's brother, David Greenglass, worked on the Manhattan Project and was persuaded by his sister in 1944 and 1945 to provide details about the atomic bomb to her and her husband, Julius, an electrical engineer discharged from the Army signal corps for being a member of the Communist Party.
In 1950 they were charged with conspiring to sell the information to the Soviets and found guilty the following year, after a controversial trial filled with circumstantial evidence. They were executed in 1953.
But those examples perhaps aren't the best comparison for Rubio's statement, Thompson said.
"It's not implausible that Sen. Rubio is right about this," he said. "One difference between Snowden and previous episodes of treason seems to be the sheer abundance of the highly secret material he was able to steal and divulge in a relatively short period of time."
In that respect, the Chelsea (formerly Bradley) Manning Wikileaks saga may be a better comparison.
Manning was convicted last year of stealing some 750,000 classified documents and videos pertaining the Iraq and Afghanistan wars and disseminating them via the online portal Wikileaks. Manning's goal was to expose possible abuses by U.S. forces.
Manning, who now identifies as transgender, is serving 35 years in a military prison.
Ted Bromund, a senior research fellow at the Heritage Foundation, said the Manning leak was likely less damaging than Snowden's revelations because the information Manning stole will eventually be largely declassified in time.
"That cannot be said of the NSA leaks," he said. "So while this is not the sort of matter that can be proven definitively, Sen. Rubio's position is sensible."
Cameron University history professor Lance Janda says we can't judge Rubio's rhetoric based on the facts in the public record.
"The verdict is still out on Snowden. We may find out in the future that his actions were even more damaging than we realize at this point," Janda said.
Read more at PolitiFact.com.