What to know about the president's 'designated survivor'

— -- On Tuesday, most of the nation's political elite -- from Vice President Mike Pence to House Speaker Paul Ryan -- will file into the House chamber to hear President Trump outline his national agenda. But one member of the administration definitely won't be watching in person.

During major presidential addresses, the administration isolates one cabinet-level official in an undisclosed location. That person takes control if a disaster were to wipe out all those in the presidential line of succession.

Usually selected by the president's chief-of-staff, the identity of the so-called "designated survivor" is kept secret until shortly before the event.

If the president dies or is removed from office, he's succeeded by the vice president, followed by the speaker of the House and the president pro tempore of the Senate, currently Utah Republican Orrin Hatch. According to the Presidential Succession Act of 1947, the president pro tempore is followed, in order, by the secretaries of state, treasury, and defense, the attorney general, and the secretaries of the interior, agriculture, commerce, labor, health and human services, housing and urban development, transportation, energy, education, veterans affairs and homeland security.

According to historians, the practice dates back to the 1960s, when the nation, rocked by the Cold War, began fearing a nuclear attack. It was not until the 1980s, however, that the survivors' identities became matter of public record.

Prior to the attacks on the Twin Towers on Sept. 11, the designated survivor had a relatively relaxed evening. One survivor recalled spending the night with his daughter, while another hosted a pizza party in the White House.

But post-9/11, security was beefed up: the designated survivor now undergoes hours of briefings and even practices disaster scenarios. Shortly before the president's speech, the designated survivor is whisked out of the nation's capital, accompanied by presidential-level security and a military aide carrying the "football," a briefcase that houses the nuclear launch codes.

During President Trump's inauguration in January, then-President Obama's secretary of homeland security, Jeh Johnson, served as the designated survivor.