As President Donald Trump resists demands from Congress that he release his federal tax returns, state legislatures are working to bar future presidential candidates from their ballots if they don’t comply.

Legislators in 18 states, including Minnesota, have introduced bills that would require presidential candidates and running mates to make their taxes public, according to the National Conference of State Legislatures.

Minnesota’s bill, introduced by House Democrats, didn’t advance. There was no companion measure in Senate, which Republicans control.

Washington is among a few states where the bills have passed in one chamber. Its Senate voted 28-21 last month to send the bill to the House.

Its sponsor, Democrat Sen. Patty Kuderer, a former Minnesotan, said in an interview that transparency is the goal. Voters, she said, deserve to know if candidates have “any potential conflicts of interest [or] ties to foreign corporations or countries.”

Although the pending legislation was largely prompted by Trump’s refusal to release his tax history, Kuderer noted that until recently U.S. Sen. Bernie Sanders of Vermont had not disclosed his full returns.

“This isn’t a Republican or Democratic issue. This really is an American issue, and it’s about having sufficient information for voters,” she said.

Since Richard Nixon in the 1970s, every president except Trump has shared tax details.