A constitutional issue largely overlooked by modern historical scholars involves the impeachment trial of President Andrew Johnson in 1868.  Having been impeached by the House of Representatives, Johnson was tried by the United States on 11 articles of impeachment.

Ultimately acquitted by a single vote on May 16, 1868 (a two-thirds majority of the Senate being required for conviction, Johnson "won" the vote, 35-19) of three of the charges against him, the Senate chose not to pursue the matter further - dropping the remaining eight counts.  So what was the issue most constitutional and historical scholars of the 21st century tend to gloss over when reviewing the sordid history of this debacle?

The case of Ohio Senator Benjamin Franklin Wade (pictured at bottom right).  With no Vice President in office during the trial (Johnson having succeeded to the presidency following the assassination of Abraham Lincoln), Wade - as President pro tempore of the Senate - was next in the presidential line of succession.  Wade was among those who voted for Johnson's removal from office, and as such, had Johnson been convicted by a 36-18 margin instead of acquitted by a 35-19 vote, Benjamin Franklin Wade would have entered the White House as Acting President of the United States as the result of his own vote.

This fact (and the dangers of permanently subordinating the presidency to the legislative branch) may not have been lost on Senator Edmund Ross of Kansas, who had been "undecided" on conviction but voted for acquittal - the ultimate "swing vote."  The crisis had been barely averted, but not forgotten:  within a year both Johnson and Wade would be out of their respective offices, and at the end of his term, Ross was tossed out of the Senate as well.