Derek Muller has posted this draft on SSRN. Here is the abstract:
States are considering legislation that would exclude presidential candidates from appearing on the ballot if they fail to disclose their tax returns. These proposals exceed the state’s power under the Elections Clause and the Presidential Electors Clause. States have no power to add qualifications to presidential or congressional candidates. But states do have constitutional authority to regulate the manner of holding elections and to direct the manner of appointing presidential electors. Manner regulations that relate to the ballot are those that affect the integrity and reliability of the electoral process itself or that require a preliminary showing of substantial support. In other words, they are procedural rules to help voters choose their preferred candidate. Tax disclosure requirements are not procedural election rules, which means they fall outside the scope of the state’s constitutional authority to administer federal elections and are unconstitutional.