FLAGSTAFF, Ariz. – The nation’s leaders came together Tuesday night for the State of the Union address, but if disaster were to strike on Capitol Hill, the government was prepared to keep running with a designated survivor.

As the nuclear threats of the Cold War era loomed over the nation, its leaders prepared to keep the government running in case of disaster.

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The practice, made public in the '80s, dates back to at least the 1960s. Should calamity happen, wiping out the president and his line of successors during the State of the Union address, one person -- the designated survivor -- is ready to take over as head of the United States of America.

This person must be in the president’s cabinet and eligible to hold office as head of the nation. Transportation Secretary Elaine Chao, for example, is out of the running because she was not born in the U.S. One of the other 23 cabinet members had to sit out in a secure and undisclosed location.

This year, Agriculture Secretary Sonny Perdue was the designated survivor.

Past chosen ones include former Housing and Urban Development Secretary Shaun Donovan in 2010, and then-Secretary of State Hillary Clinton. Though Donavan was the official designated survivor, both missed the State of the Union and Clinton would have been next in the presidential line of succession.

The role isn’t limited to the State of the Union address.

Last year, when Trump addressed his first joint session of Congress, Veterans Affairs Secretary David Shulkin filled the position.