The Near-Coup of 1868
When we think of constitutional crises in American history, the Watergate Affair and its aftermath usually first comes to mind. The spectre of a President of the United States having participated in criminal activity, coupled with the likelihood of impeachment and removal from office was very real in 1974. While Richard Nixon's resignation ended that crisis, one before it could've had more significant impact..
The Impeachment of Andrew Johnson
While Nixon avoided impeachment by resigning, our 17th President, Andrew Johnson, would not. Impeached by the House of Representatives in 1868, Johnson was tried by the Senate on eleven articles. Ultimately acquitted on three of the articles that May, the Senate chose to drop the remaining eight counts. But on one of the articles, the vote to convict was 35-19 - just one vote short of kicking Johnson to the presidential curb.
So aside from the fact that we came one vote away from removing Johnson from the White House, why was this a potential Congressional coup d'etat? Because of the man who would've replaced him had that vote not gone Johnson's way.
Acting President Benjamin Franklin Wade
As Johnson himself had ascended to the presidency following the death of Abraham Lincoln, there was no Vice President in office during Johnson's impeachment trial.
Ohio's Benjamin Franklin Wade, then President Pro Tempore of the United States Senate, was next in the line of succession. Wade was among those who voted to convict Johnson. So, had Johnson been convicted by a 36-18 vote rather than acquitted by a 35-19 score, Wade not only would have entered the White House as Acting President - he'd have done so as the result of his own vote.
The potential ramifications of this act, essentially shattering the separation of powers and subordinating the executive branch to the legislative, may not have been lost on Senator Edmund Ross of Kansas.
Throughout the impeachment trial, Ross had been undecided on conviction. At his moment of truth, Ross chose to vote to acquit, and a potential constitutional crisis had been averted. But the matter hadn't been forgotten by their respective constituents - Johnson, Wade and Ross each would be out of office at the ends of their respective terms.