(CNN) As President Donald Trump continues to resist requests for his tax returns from Democrats in Washington, state lawmakers are moving to require him to release his financial information if he wants to run for president again in 2020.

Lawmakers in 18 states across the country, including New York, Illinois and Washington, have introduced bills that would require all presidential candidates and their running mates to release their individual tax returns in order to qualify for the presidential primary ballot, according to the National Conference of State Legislatures.

The latest is in Illinois, where state senators approved a bill this month that would require presidential candidates and their running mates to release personal tax returns for at least the five most recent years in exchange for access to the state's 2020 ballot. It's unclear whether the measure will succeed in the Illinois House or if the state's Democratic Gov. J.B. Pritzker would sign the bill if it passes.

Trump is the only President in modern history who's refused to voluntarily release his tax returns , and also to maintain his interest in his personal business while in office, prompting questions and lawsuits about potential conflicts. Several Democratic 2020 candidates have already released their tax information , seeking to draw a contrast with Trump.

On Tuesday, the Trump administration again blew past a deadline from House Democrats for six years of Trump's tax returns, which they have requested under an obscure anti-corruption statute. Instead, Treasury Secretary Steven Mnuchin wrote he would respond to House Ways and Means Committee Chairman Richard Neal's request by May 6, after the Justice Department concludes its legal review.

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