One of the big things about the State of the Union address is talking about which Cabinet Secretary has been sent to an undisclosed location in case the Capitol Building goes up in a cloud of neutrons and there is no one left to run the government.
Last night that honor went to the Secretary of Agriculture, Tom Vilsack who was a former Governor of Iowa.
According to the Presidential Succession Act of 1947, after the Vice President, Speaker of the House, and President Pro Tempore of the Senate, the order of succession to the Presidency is:
Secretary of State, Secretary of the Treasury, Secretary of Defense, Attorney General, Secretary of the Interior, Secretary of Agriculture, Secretary of Commerce, Secretary of Labor, Secretary of Health and Human Services, Secretary of Housing and Urban Development, Secretary of Transportation, Secretary of Energy, Secretary of Education, Secretary of Veterans Affairs, Secretary of Homeland Security.
Which are more-or-less in the order the Cabinet Department was created.
When the Department of Homeland Security was stood up on March 1, 2003 there was some discussion as to whether the order of succession should be changed to put the DHS Secretary ahead of Interior, Ag, Commerce, HHS, HUD … and all those other Cabinet Secretaries who are responsible for seeds and rental units.
Alas, 18 senior government officials - including the President - have got to meet their makers before the Secretary of DHS can order new drapes for the Oval.
The Constitutional basis for a State of the Union address is found in Article II, Section 3, Clause 1:
[The President] shall from time to time give to the Congress Information on the State of the Union, and recommend to their Consideration such measures as he shall judge necessary and expedient.
Until the 1930s the address was simply known as the Annual Message. In 1942 it became known as the State of the Union address.
According to the Clerk of the House's webpage:
In the 19th century, the annual message was both a lengthy administrative report on the various departments of the executive branch and a budget and economic message.
After 1913, when Woodrow Wilson revived the practice of presenting the message to Congress in person, it became a platform for the President to rally support for his agenda.
Until 1934, the Annual Message was delivered every December.