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When US President Joe Biden steps onto the dais in the US Capitol for his State of the Union address on Tuesday, a "who's who" of Washington power players - lawmakers, Supreme Court Justices and top military brass - will be just feet away.

But one player will be notably absent - the so-called "designated survivor" tasked with taking charge in the event an unforeseen tragedy wipes out or incapacitates the rest of the government officials gathered for the speech.

Labour Secretary Marty Walsh will be this year's designated survivor. Last year, it was Commerce Secretary Gina Raimondo.

According to the non-profit National Constitution Center, the tradition began in the 1950s, when the spectre of a possible nuclear war between the US and the Soviet Union loomed in the early Cold War.

It was only in 1981 that the US government first publicly identified the designated survivor. That year, it was Terrel Bell, then President Ronald Reagan's secretary of education.

Since then, designated survivors have been chosen from a range of government agencies, including the attorney general in the justice department, and a number of cabinet-level secretaries.

Under former President Donald Trump, the State of the Union designated survivors were the secretaries of agriculture, energy and the interior.

For the traditional presidential address to a joint session of Congress that followed his election, Mr Trump's designated survivor was Secretary of Veteran Affairs David Shulkin.

Designated survivors must be eligible for the presidency to be named to the position and will only take on the role of commander in chief if every higher-ranking official in the line of succession is incapacitated.

Eligibility requirements demand the person must have been born a US citizen, meaning naturalised citizens like current Homeland Security Secretary Alejandro Mayorkas and Energy Secretary Jennifer Granholm cannot take on the position.

The role has also been thrust into the public consciousness in part by Hollywood.

Designated Survivor, a TV show, has actor Kiefer Sunderland playing the Secretary of Housing and Urban Development and taking the helm of the US government after an explosion destroys the Capitol during the State of the Union.

The reality, however, is far more mundane. Last year, Ms Raimondo watched the speech from a secure location, but otherwise had a normal day.

In a 2017 Politico essay, one of Bill Clinton's designated survivors, former agriculture secretary Dan Glickman, recalled spending the State of the Union at his daughter's apartment in New York, alongside a military officer carrying the codes necessary to launch the US nuclear arsenal.

"I sometimes wonder if I would have had the courage to give the order," he wrote.