On January 4, U.S. Senator Ron Wyden (D-Oregon) introduced S.26, which requires major party presidential candidates to file a copy with their federal income tax returns with the Federal Election Commission, and also requires sitting presidents to release their tax returns. Here is the bill. It has ten co-sponsors.
“Major party” presidential nominees are those whose party received at least 25% of the vote in the previous presidential election.
If the candidate does not release the tax return, the FEC will then ask the IRS to release it to the FEC, so that the public can see it. This is a far better approach than using ballot access laws to compel release of the return.