What It's Like to Be the President-in-Waiting on the State of the Union Night

— -- Almost all our nation’s leaders, from the justices of the Supreme Court to the top brass of the U.S. military, will file into the chamber of the House of Representatives tonight to watch President Obama’s State of the Union address.

But while his colleagues enjoy the glitz and glamour at the Capitol, one lone cabinet secretary will be left behind, squirreled away in an undisclosed location, poised to become president if a calamity were to wipe out everyone in the chamber.

Because of security concerns, the “designated survivor” is not announced until shortly before the speech. But according to interviews with former designees, it’s a sobering job.

“It really focuses your mind on just how precious and vulnerable, really, our country is,” former Veterans Affairs Secretary Jim Nicholson told ABC News.

Nicholson was tapped to be designated survivor during President Bush’s 2006 State of the Union address. “What an awful situation it would be for the country if I became president,” he said.

“The only reason you’re there is a failsafe,” former Interior Secretary Dirk Kempthorne, the designated survivor during the 2008 State of the Union, said. But if the unthinkable happens, “you better be prepared physically, mentally and spiritually.”

And training to be commander-in-chief is no easy task.

Chosen by the White House chief of staff well in advance of the speech, these virtual presidents-in-waiting go through hours of logistical and legal briefings, role-playing disaster scenarios and even outlining the speech they’d make they if they assumed office, according to Nicholson.

The specifics remain classified, but Nicholson says it’s an extensive undertaking, with staff referring to the designated survivor as “Mr. President.”

“We did drills,” he says. “We would role-play it -- and in that role, I was president.”

On the day of the speech, designated survivors are whisked out of Washington via helicopter to a secret spot where, according to Kempthorne, a countdown begins before the designated survivor is officially on standby.

“There’s actually a large clock which everyone can see which is a countdown before POTUS enters the House chamber,” Kempthorne says. “I remember going through and asking a series of questions that I knew it would be too late in the game” to ask if disaster struck.

Surrounded by Secret Service and White House personnel, the designated survivor is then given a series of security briefings and served a “delicious” (that’s how Nicholson described it) dinner prepared by the White House mess.

It’s a heady experience, but a sobering one as well.

“Now in this era of terrorism and the possibility of the use of weapons of mass destruction by stateless terrorists, there’s a lot of reality in this,” Nicholson said. “The security was ever-present and adequate and very expert.”

He added, “When we got the call from the White House saying, ‘the President is closed into the White House, all is clear, you can go home,’ people weren’t calling me Mr. President anymore.”