Have you read Tom Clancy's book, "Executive Orders"?? Ryan is appointed to the Vice Presidency because the current VP had resigned and literally within minutes, became President when a suicidal Japanese pilot flew an airliner full of fuel into the Capitol building, killing the President and most of the House and Senate
The leader of Iran launched a biological attack against the US with a weaponized Ebola virus. To combat the spread of Ebola, he orders the shut down of all transportation systems, tells people who aren't in essential occupations to stay home and mobilizes the National Guard to close all the roads. Here is the important part. When told that he couldn't do that under the Constitution, the response is, "The Constitution never envisioned this kind of a situation and does not cover it. It is a political document, not a suicide pact"