President Wade: One Vote Shy
Jefferson, Ohio
President Andrew Johnson was impeached by the U.S. House of Representatives in 1868, put on trial in the Senate, and survived by a single vote. Had only one Senator switched, Johnson would have been out, and, since Johnson had no Vice President, the President Pro-tempore of the Senate would have been in. That was Benjamin F. Wade.
(The current presidential line of succession, which puts the Speaker of the House of Representatives third in line, was only put in place in 1947).
Some speculated that Johnson was spared because his enemies in the Senate hated Wade even more than they hated Johnson.