"Because impeachment of a President is a grave step for the nation, it is to be predicated only upon conduct seriously incompatible with either the constitutional form and principles of our government or the proper performance of constitutional duties of the presidential office." --Constitutional Grounds for Presidential Impeachment, 1974
Before 1974, the last time Congress impeached a president was with President Andrew Johnson in 1868. Therefore, since there was little understanding of how to impeach President Richard Nixon, the US House Committee on the Judiciary assembled a staff of lawyers to research the history of impeachment and how this could be applied to Nixon. The result of this investigation was Constitutional Grounds for Presidential Impeachment--Report by the Staff of the Nixon Impeachment Inquiry, which with its the definition of "high Crimes and Misdemeanors" became an indispensable guide for impeachment. An interesting twist of history is that one of the staffers who helped write this report was Hillary Rodham Clinton, who subsequently experienced the impeachment of her husband and in 2019 has a vested interest in the potential impeachment of President Trump.
An updated report, Impeachment and Removal, without the historical context, was released in 2015 (also available from Cosimo Reports.)