Sen. Ron Wyden (D-Ore.) wants a law to force presidential candidates to release at least three years’ worth of tax returns. (Photo by Chip Somodevilla/Getty Images)

The Senate’s sharpest-tongued privacy advocate has proposed a law to force presidential candidates to release their tax returns within days of securing their party’s nomination.

Oregon Democrat Ron Wyden’s bill would require presidential candidates to release at least three years worth of their tax returns within 15 days of being officially nominated at their party convention, applying a legal requirement to something Wyden argues has been standard practice for over 40 years.

“If you are a major party’s nominee to be the leader of the free world, Americans have said ever since Watergate that you don’t get to hide your tax returns,” Wyden said. “It ought to be the law.”

Whether presidential candidates should release their tax returns is a hot issue on the campaign trail after Donald Trump, the GOP’s presumed nominee, began backing off earlier indications that he would release his returns. At first, Trump argued that he would wait until the IRS was done auditing him; then, he said there was nothing to learn from his returns anyway; recently, he’s even told people asking that the returns are “none of your business.”

The issue was also a big one for 2012 Republican nominee Mitt Romney, also a wealthy businessman who delayed releasing all of his tax information but eventually disclosed it in September 2012.

Wyden, the senior Democrat on the committee with jurisdiction over tax issues, and a supporter of presumptive Democratic nominee Hillary Clinton, suggested that such concerns were part of what motivated him to file the bill. Wyden said he has not talked to Clinton, who has been demanding Trump release the returns. Clinton herself released eight years of tax returns in summer 2015.

“For literally four decades now, Democrats, Republicans, regardless of party, regardless of political philosophy have made this information available,” Wyden said.

“I don’t believe the public should have to believe the boasting or take somebody’s word for it,” Wyden argued. “Nominees have traditionally released a lot more – [three years] ought to be used as a starting point.”

Trump has said both that he has paid a “substantial” amount of taxes and that alleged that the wealthy businessman tries “very hard to pay as little tax as possible.” Based on three tax returns form the late 1970s and 1981, the Post has reported that Trump paid $0 in taxes some years.

Wyden said the purpose of asking to see tax returns is “not just about what rate you pay,” but to get a read on a candidate’s values as well.

“Do you even pay taxes, do you give to charity, are you abusing loopholes at the expense of middle class taxpayers, are you keeping money offshore?” Wyden listed as just some of the questions that publicizing tax returns could answer.

Wyden does not yet have any GOP co-sponsors for his bill.