The Speaker of the House is the highest-ranking member of the House of Representatives in the United States, and the second in the presidential line of succession (with the Vice President first in line).
Day 4 Edit
Don Ashton, the Speaker during Day 4, was brought to the White House bunker to safely assist with the terrorism crisis of the day. President Charles Logan, who had just been sworn in, made decisions which made Ashton doubtful of his leadership abilities, and he was openly hostile to some of Logan's ideas. President Logan and his advisor, former President David Palmer, staged a conflict so that Logan could appear to retake command (and Ashton's side).