WASHINGTON ― With less than eight weeks to go before the elections, Sen. Elizabeth Warren (D-Mass.) called Donald Trump “chicken” on Thursday as Democrats ramped up the battle over the Republican nominee’s refusal to release his tax returns.

Warren assailed Trump after Senate Democrats tried to force a vote on legislation that would require all presidential nominees for a major political party to disclose their tax returns.

“Donald Trump makes a big show of strutting around pretending to be tough, but he’s too chicken to show his tax returns to the American people,” Warren said on the Senate floor. “He’s had a million excuses, but we all know why Donald Trump isn’t releasing his taxes: He’s hiding something.”

Trump often cites an IRS audit as the reason he will not make his tax returns public, though he admits he could “immediately” release them if he felt like it. On Wednesday, Donald Trump Jr. said his father can’t release his tax returns because they would raise too many questions that would take away from the campaign’s message.

Democratic presidential nominee Hillary Clinton and her husband, former President Bill Clinton, have made their tax returns available dating back to 1977.

Warren wondered aloud how Trump’s tax returns could be any worse than the Republican nominee’s repeated praise of Russian President Vladimir Putin.

“Donald Trump praises brutal dictators and murderers,” Warren said. “He threatens our allies, he denigrates democracy right here at home. So what is so bad that Donald Trump has to hide it? Would his tax returns show how deeply Donald Trump’s personal financial interests run counter to the national interests of the United States of America?”

Warren’s speech came moments after Sen. Ron Wyden (D-Ore.) tried to bring his bill ― the Presidential Tax Transparency Act, which mandates candidates disclose at least three years of their tax returns ― up for a vote.

“Since Watergate, there has been a bipartisan tradition honored by all candidates that they would release their tax returns,” Wyden said on the floor. “Every Democrat, every Republican, every liberal, every conservative has subscribed to honoring this particular tradition.”

The tradition Wyden referred to dates back to 1973, when the IRS audited President Richard Nixon over questionable donations and a belief that he had cheated the tax system. Though Nixon didn’t release his returns until after he was re-elected, every Republican nominee since President Ronald Reagan in 1980 has voluntarily done so.

He’s had a million excuses, but we all know why Donald Trump isn’t releasing his taxes: He’s hiding something. Sen. Elizabeth Warren (D-Mass.)

As lawmakers headed for the exits after four days of minimal work, Wyden asked for unanimous consent that his legislation be considered by the upper chamber, allowing it to bypass the committee process. Senate Majority Whip John Cornyn (R-Texas) objected.

“If my friend from Oregon wants to discuss transparency and bring the presidential election here to the floor of the United States Senate, I think the person we should start with is the former secretary of state,” Cornyn said, referring to Clinton’s private email sever.

Cornyn then offered to speed up consideration of of his legislation ― one that would revoke the security clearance of anyone found to be careless in handling classified information ― alongside Wyden’s bill.

Wyden shot it down. “[Cornyn’s] attempt to hide the dishonoring of a tradition of openness and accountability behind a political witch hunt ought to tell Americans all they need to know about Senate Republicans,” he said.

“Once you break from the tradition, it will be hard to get it back,” Wyden told reporters Thursday after trying to force the vote.

Sen. Chris Murphy (D-Conn.) joined Wyden at a press conference after the Democrats’ failed attempt and questioned Trump’s motives for his presidential run.

“Are you running to protect our national security interests or are you running to protect your family’s financial interest? And we can’t know the answer to that question without these tax returns,” Murphy said. “Trump’s son has been very clear that the family has massive financial interests in Russia. He’s said it.”

In 2008, Trump Jr. reportedly said, “Russians make up a pretty disproportionate cross-section of a lot of our assets,” according to trade publication eTurboNews.

“It would be embarrassing to find out that Donald Trump didn’t pay any taxes,” Murphy added.

It’s unlikely Republican leaders will allow Wyden’s measure to come up. Senate Majority Leader Mitch McConnell (R-Ky.) has said Trump should release his tax returns, but doesn’t think there should be a bill forcing such disclosure.

Democrats won’t have many more chances to force a vote on the bill. Congress is set to leave for a break in October ahead of the elections.

Still, Wyden said he would continue to push for a vote and talk to his Republican colleagues about signing onto his bill, especially those who have been critical of Trump.

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.