Nixon had his people break into the Watergate to steal documents from Democrats. That walks and quacks like a crook...
The factual historical record is clear and not in dispute - Nixon is not known to have any prior knowledge of, or involvement in, the Watergate break in. He was not informed of it until after it occurred.
If there was any evidence to show he had been involved in planning the actual break in, or had prior knowledge, you can be sure democrats would have included that charge in the articles of impeachment. But such a charge is not there.
You can also be sure that if there was any testimony or evidence of a priori knowledge or involvement the charge would have been made and documented by Bob Woodward and Carl Bernstein.
Nixon was rightly accused of many things, but directing the Watergate Plumbers to break into the Watergate to steal documents from Democrats is not one of them.