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 U.S. Senate, and survived by a single vote. Had only one Senator switched, Johnson would have been kicked out of office and, since Johnson had no Vice-President, the President Pro-tempore of the Senate would have been the new U.S. President. 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 into place in 1947).
Wade was one of the Senate's most outspoken liberals, and it was generally accepted that Andrew Johnson was spared because Wade's fellow Senators hated him even more than they hated Johnson.