Sen. Chris Murphy, D-Conn., said Thursday he's in favor of a Democratic-led piece of legislation that would effectively force Republican presidential nominee Donald Trump to release his tax returns.

"The American people deserve to know if the president of the United States is making decisions in order to advance the best interests of the citizens of this country or the best interests of his wallet," Murphy said on a conference call with U.S. Sen. Ron Wyden, D-Ore., who introduced the Presidential Tax Transparency Act.

Wyden's bill -- which has gotten little traction in the Republican-controlled Senate -- would alter existing election law to require major party presidential nominees to file three years of tax returns with the Federal Elections Commission. If a candidate refuses, the legislation directs the Treasury Department to release the documents.

Trump has come under scrutiny for refusing to release his tax returns, something presidential candidates have a long histor yof doing. His Democratic opponent, Hillary Clinton, who released her own returns last week, has seized on the issue and began running TV ads this week featuring old footage of Trump saying he would “absolutely” release his tax returns if he ran for public office.

Some Republicans have also called on Trump to release the documents, including U.S. Rep. Mark Sanford, R-S.C., who wrote an op-ed in the Sunday New York Times titled “I Support You, Donald Trump, Now Release Your Tax Returns.”

Trump has said he won’t release his recent returns until the Internal Revenue Services completes an audit. Critics speculate that Trump is delaying the release because the documents could show that he pays little or no taxes, makes few charitable donations, isn’t as wealthy as he claims or that he has substantial foreign investments.

“The worry here is that Trump’s positioning on Russia – which is way out of the foreign policy mainstream – is a play to make himself and his family richer,” said Murphy, who is the ranking member of the Senate Foreign Relations Committee. “It stands to reason … that Trump could be looking to the presidency as the ultimate means of increasing his wealth.”