Last year, Whet Smith and Mark J. Rozell wrote at The Hill:
One simple change Congress should make is to revisit the Presidential Succession Act. Currently, the next two figures in line to fill a presidential vacancy are the speaker of the House and the president pro tempore of the Senate. This is decidedly less than ideal.
First, a member of Congress derives his or her authority from a regional mandate. Second, it creates major separation of powers issues. Third, it overturns a national electoral mandate if the House speaker is from the opposition party. Fourth, the president pro tempore in the modern era is selected essentially for being elderly, which is not a great characteristic for an emergency replacement.