WASHINGTON - U.S. Rep. John Kline was not on hand for Tuesday night's State of the Union address. The Minnesota Republican was not boycotting President Obama. He was his potential successor.

According to Kline spokesman Troy Young, the congressman "was asked by security officials to view the proceedings from an alternate site close to the Capitol." That's all Young said he was authorized to say.

But the clear implication, which congressional aides are barred from confirming, is that Kline was a "designated survivor" in the event of a catastrophic event that decapitated the government.

It's a Cold War practice that has been extended in recent years from a single Cabinet member to members of Congress.

Under the U.S. Constitution, House Speaker John Boehner would be second in line to the presidency, after Vice President Joe Biden. Next would come the president pro tempore of the Senate and the secretary of state.

But should they and all others in the line of succession be killed or incapacitated, modern practice has been to designate a surviving senator and House member to provide for the continuity of government. They would also ascend to the positions of speaker and president pro tempore, putting them in the line of succession.

So, had the worst happened on Tuesday night, Kline, a close ally of Boehner, could conceivably have had a path to acting president by standing in as House speaker.

While the scenario is hard to imagine, Kline would be no stranger to executive power. As a decorated Vietnam-era Marine colonel, Kline served as a senior military aide flying Marine One for President Jimmy Carter and carrying President Ronald Reagan's "football" containing the nation's nuclear attack codes.

Kevin Diaz is a correspondent in the Star Tribune Washington Bureau.