The election for the president is the Electoral college. You are talking about how a state chooses its electors, and this is a matter for the state.
The people in Maryland get to choose between the various candidates who have qualified to be on the ballot. The winner of the election in Maryland gets to choose who he or she wants to represent Maryland in the electoral college. This is all controlled by State Law. The constitution just requires that states choose electors.
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress:
The Congress may determine the Time of choosing the Electors, and the Day on which they shall give their Votes; which Day shall be the same throughout the United States.
Note that this doesn't even mandate presidential elections. It would be constitutional for the legislature of Maryland to simply ask the Governor to pick 10 people.
The only other appropriate part of the constitution is the 24th amendment
The right of citizens of the United States to vote in any primary or other election for President or Vice President, for electors for President or Vice President, or for Senator or Representative in Congress, shall not be denied or abridged by the United States or any State by reason of failure to pay any poll tax or other tax.
However, this is about the right to vote, not the right to stand.
In short, the constitution allows the states wide freedom to run presidential elections in the manner that they see fit. The choice of who appears on the ballot is a matter for Maryland Law. It would be possible to challenge the law in the Court of Appeals of Maryland. But there does not appear to be a breach of the US constitution in asking for tax returns.