These were three of the biggest scandals in modern American history, and party loyalty stayed strong almost through the end of each.

The second trend is that there is an ideological component to party loyalty during scandal. In the simplest sense, more moderate members (in comparison to Congress overall) are more likely to split from a president of their party. Let’s look at the same votes and decisions as above for Watergate and Iran-contra, as well as the vote on the initial impeachment investigation of Clinton’s conduct. (To make a more apples-to-apples comparison, since Nixon and Reagan were never formally impeached.) According to DW-Nominate scores, which measure the ideology of each member of Congress based on roll call votes from previous congresses, the few Republicans who broke with Nixon4 and Reagan were more liberal than the rest of Republicans in Congress then. Similarly, the 31 Democrats who joined Republicans to support the initial impeachment investigation of Clinton’s conduct were generally more conservative than the 175 Democrats in Congress who opposed that GOP proposal.