Not just the Dems. 6 out of 17 Republicans on the House Judiciary Committee also voted to impeach Nixon. Generally when a politician’s own party members turn on him, he’s guilty.
There is also the fact that he resigned rather than face impeachment proceedings. Now why would he do that if he had nothing to hide?
Nixon may have had some good policies, but that doesn’t change the evidence that points towards his committing impeachable offenses.
He belonged to a party comprised of spineless wimps;then and now.