I think you would be walking on pretty thin ice to go forward with something that wasn't an abuse of the office itself. Treason, bribery and high crimes and misdemeanors does not mean "crimes." It has elements of using the office for inappropriate ends. And anyone can call up someone and ask for help in an election -- Warren could do that today. What makes it a high crime is flexing the power he has because of the office to aid in it. In this case, it is withholding the funds or pressuring related to the funds.
There are of course felonies that would be impeachable themselves. "Shooting someone on 5th Avenue," etc. The campaign finance violation might qualify, but it would be a tougher argument to make. (Note the weird effect of just saying "felony does the trick": You don't want to basically put in the hands of Congress the right to write statutes to post-hoc change the meaning of the constitution, which would be the case since they define what is a felony).
Hugo Charles Black wrote a good book on it from Nixon's time and Phillip Bobbitt has an update for the present. It goes through the whole process and includes a pretty in depth look at what the debate was all about in 1789. It's pretty excellent.