How specifically? Parties can set requirements to be on their ballot, states conduct election. I would like to think courts would stop this but not very confident in that since we are talking about Trump.
“Parties can set requirements “
It’s not the party setting the requirement, it’s the state.
“No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.”
ARTICLE II, SECTION 1, CLAUSE 5
Don’t see anywhere here anything about tax returns.