Democrats on the House Ways and Means Committee tried to frame the issue as a matter of national security. They questioned whether Trump has any investments in Russia.
Trump has said he has no investments in Russia, and Democrats acknowledged that have no evidence otherwise. That's why they want Trump's returns, said Rep. Joseph Crowley, R-N.Y.
The move came a day after Trump's national security adviser, Michael Flynn, was forced to resign over conversations he had with Russian officials before Trump took office.
"People are concerned about Michael Flynn putting himself in a position to be blackmailed. What position is the president in?" asked Crowley. "Our national security is at risk."
The Ways and Means Committee has legal authority to obtain confidential tax records. The committee could then vote to make them public.
On Tuesday, committee Democrats tried to amend a routine annual oversight plan to insert a provision that called for obtaining Trump's tax returns. Republicans blocked it on a straight party-line vote, 23-15.
Committee Chairman Kevin Brady, R-Texas, said the panel should only use its power to ensure that tax laws are being administered properly — not to investigate the president.
"As chairman of this committee I will not allow Washington to return to the bad old days when government officials used their powers to intimidate, harass and destroy their political enemies," Brady said. "If Congress uses its power to rummage around the president's tax returns for political purposes, what's prevents it from doing the same to average Americans?"
Most famously, former President Richard Nixon was accused in his articles of impeachment of illegally obtaining tax records of his political enemies, and of ordering audits. Since then, federal law was changed so that neither the president nor the IRS commissioner can single out an individual for an audit.
Shunning decades of tradition, Trump has steadfastly refused to release his tax returns or to divest from his business. Instead, Trump said he has turned operation of his business over to his sons, while still retaining his stake.
Democrats tried to make Trump's taxes an issue during the presidential election, questioning his wealth and whether he generously donates to charity, as he claims. After the election, the head of the Office of Government Ethics scolded Trump for not divesting from his business, saying it is the only way to avoid conflicts of interest.
Trump adviser Kellyanne Conway has said people don't care about the president's taxes.
Trump's tax returns would reveal a trove of information about his business dealings and his sources of income. They wouldn't show his overall net worth. But they would show his annual income and where it came from.
"No one on this committee can say for sure that Trump doesn't have investments in Russia," said Rep. Bill Pascrell Jr., D-N.J. "We don't know."
Pascrell promised to raise the issue "over and over and over again."
"I'm good at that," he said.