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For a few fleeting hours two decades ago, then-Agriculture Secretary Dan Glickman was Washington's designated survivor.

If a catastrophic event had wiped out a packed House chamber during President Bill Clinton's State of the Union, Glickman — waiting in the wings outside the capital — would have become the acting commander in chief.

"You would have had a President Glickman," he said.

His potentially life-altering appointment as the designated survivor comes from tradition — one that will carry on Tuesday night as President Donald Trump gives his first State of the Union address.

Agriculture Secretary Sonny Perdue J. Scott Applewhite / AP file

For 2018, the designated survivor was Agriculture Secretary Sonny Perdue, which the White House revealed just hours before the political pageantry on Capitol Hill was set to begin.

Last year, when Trump addressed his first joint session of Congress, Veterans Affairs Secretary David Shulkin was tapped as designated survivor and sent to a secure and undisclosed location.

While the process of selecting someone may seem cloaked in mystery — though it has found new popularity thanks to a TV show of the same name — members of the designated survivor club peeled back the curtain to NBC News.

"One part of it seems totally unrealistic. Then there's a certain realism that sets in and you have to take it seriously," said former Veterans Affairs Secretary Jim Nicholson, who was the designated survivor when President George W. Bush gave his State of the Union in 2006.

Nicholson said Bush's chief of staff, Andy Card, asked him weeks in advance to be the designated survivor. It only became known publicly on the day of the address.

That night, a helicopter flew him to a hidden meeting place where he was briefed about his duties in a drab command center-like room. For dinner, he ate a "delicious steak," he said. The TV was tuned into the State of the Union.

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Government workers gave Nicholson a rundown of what would happen should calamity strike.

"You kind of drill it and role-play ahead of time," he said. "They call you Mr. President."

He said he was chosen as a designated survivor because he was involved with the Bush administration's ongoing contingency planning, which became paramount after the events of 9/11.

How a survivor is selected

The idea of singling someone out is an artifact from another perilous period in history: the Cold War.

Records go back to at least the 1980s on designated survivors, historians say. To be chosen, the person must be in the president's cabinet in a position that is part of the line of succession and falls under the criteria to run for the office, including being a natural-born U.S. citizen and at least 35 years old.

Theoretically, any of those cabinet members or cabinet-level officers could be selected, but historically, the appointed person has come from newer departments such as veterans affairs, homeland security or energy.

Whittling down the list takes time, said Barbara Perry, director of presidential studies at the University of Virginia's Miller Center.

For starters, who in the cabinet needs to be at the particular event? Generally, a vice president would, although an exception was made for Vice President Dick Cheney in September 2001, when he was a co-designated survivor in order to keep him safe while Bush addressed Congress post-9/11.

Related: Trump's Cabinet: What You Need to Know About It

Perry said anointing someone could also come down to whether the president will be talking about a particular department in his speech, which would require the cabinet secretary to be in the audience so cameras can pan to him or her, or a cabinet member is conveniently out of D.C. at that time.

Selecting a survivor last year was tricky since a number of Trump's cabinet appointees were still awaiting Senate confirmation. In addition, Transportation Secretary Elaine Chao was out of the running because she wasn't born in America.

Power, but only for a night

What might be in store for a designated survivor? In past administrations, the person might stay in the White House or travel out of town — there's no protocol for where they have to be.

Former Veterans Affairs Secretary Anthony Principi told NBC News he was taken to a secure location in February 2001, when the recently sworn-in Bush spoke to a joint session of Congress.

"It was uneventful," Principi said of the night, which consisted of watching the address. "But that was before 9/11."

Since then, the Senate has also named its own designated survivor in order to keep that branch of government operating amid a doomsday scenario.

Former Agriculture Secretary Dan Glickman speaks at the ShoWest opening ceremony in Las Vegas on March 13, 2007. Glickman is currently the executive director of the Aspen Institute Congressional Program. Jae C. Hong / AP, file

In the run-up to the State of the Union in 1997, the White House called Glickman's chief of staff to tell him he was on the hook as designated survivor. He was given a briefing, but nothing extensive or as important as the nuclear codes, he recalled.

Glickman was permitted to stay with his daughter, who was then living in lower Manhattan.

On the night of the speech, he was swarmed by Secret Service who took him from Andrews Air Force Base to LaGuardia Airport on a small plane. They waited for him at his daughter's apartment.

After the address ended without a hitch, the Secret Service agents left. Glickman and his daughter went for a late-night bite at a Japanese restaurant 10 blocks away, and were later caught in a sleet storm without a ride home. At that point, it hit Glickman that being designated survivor had its perks — but those were short-lived.

"Only three hours before, I was potentially the most powerful person in the world," Glickman said. "Three hours later, I couldn't even get a cab."

This story has been updated from an article originally posted on Feb. 28, 2017.