WASHINGTON ― Democratic senators on Thursday proposed requiring all the senior-level appointees of President-elect Donald Trump to release their taxes ― something Trump himself, in a startling departure from the norm for presidential nominees, never got around to doing.

Arguing that Trump has done little to guard against conflicts of interest in his own dealings, Sens. Patty Murray (D-Wash.), Debbie Stabenow (D-Mich.) and Ron Wyden (D-Ore.) said that the billionaires and millionaires Trump is selecting for his cabinet need to be vetted especially carefully.

“Donald Trump campaigned on a promise that he would drain the swamp,” Stabenow told reporters.

“But it seems [he] thinks the way to do that is by filling it with even bigger swamp creatures,” Murray said.

Among the super-rich would-be government officials are hedge fund billionaire Steven Mnuchin, billionaire investor Wilbur Ross and Amway billionaire Betsy DeVos, whom Trump has put forward as his picks for treasury secretary, commerce secretary and education secretary, respectively.

Mnuchin’s nomination would go through the Financial Services Committee, and under existing committee standards he would already be required to submit three years of tax returns.

But most other committees have not typically required tax forms. When committees meet at the start of the year to pass their internal rules and budgets, the Democrats say they will offer measures to require the new disclosure in all the panels.

“Our proposal is a roadmap for each committee chair to increase the openness and transparency among senior government officials,” Wyden said. “Government officials don’t get a free pass to flout our tax laws, abuse loopholes or hide money in shelters. The president-elect’s top officials are required to resolve conflicts of interest, and I believe they ought to be able to prove they follow the nation’s tax laws just like everybody else.”

A spokesman for Senate Majority Leader Mitch McConnell (R-Ky.) did not immediately respond to a request for comment.

It’s possible, or perhaps likely, that Republicans will resist the idea, since increased disclosure could offer Democrats more information to use against Trump administration nominees.

But such records have proved important in the past. Wyden pointed out that in 2009, after Barack Obama had nominated then-Sen. Tom Daschle (D-S.D.) as secretary of health and human services, information in Daschle’s taxes led him to withdraw from consideration for the post.

The Democrats did not say how far they are willing to push to win the increased disclosure. When it was pointed out to them that Democrats could filibuster when the Senate formally votes to organize the committees at the start of the next session, they did not commit to such a blockade.

“At this point we’re talking to Republican colleagues, because this is basically in the interests of good government and transparency and accountability,” Stabenow said. “We’ll see what happens, and then we’ll make a decision from there.”