The problem seems to be...in my mind...that it is a fishing expedition....and that there is no requirement for him to hand them over when running for President.
This probably won’t stop the Dems. They don’t much care about USSC rulings they don’t like and just work around them.
The joint committee on taxation has legitimate access to tax returns. This is not what the intent of that committee is, administration of the tax law. This is actually what you say it is, a fishing expedition and worse the use of the Congress to promote a political agenda and theater against a now private individual. Roberts apparently sees the danger in this stuff if allowed to go on.