(John Locher/AP)

Until now, there's never really been a need for a law to require presidential candidates to release their tax returns: For the past four decades, every president and/or major party nominee has released their returns on his or her own. It's traditional. It's expected.

But it's not legally required. And Democrats in Congress have seized on Donald Trump's refusal to release his tax returns to try to change that — and to score some political points by daring their Republican colleagues to defy their presidential nominee by taking up this bill.

Let's briefly explain:

In May, as it became clear Trump would be the Republican Party's nominee, Sen. Ron Wyden (D-Ore.) introduced the Presidential Tax Transparency Act.

The bill would require any major party presidential candidate to share the three most recent years of his or her federal income tax returns within 15 days of being nominated. If the nominee refused, the Federal Elections Commission could go to the Treasury Department and grab his or her tax filings and make them public.

But as with most things in Washington in an election year, the bill is largely symbolic. When Democrats introduced it this spring, they knew its chances in a Republican-led Congress were nil. The House version of the bill, introduced by Rep. Anna Eshoo (D-Calif.), has the support of 31 Democrats and just two Republicans, Reps. Mark Sanford (S.C.) and Walter B. Jones (N.C.)

This is just not a fight Republicans want to pick with their nominee right now. House Speaker Paul D. Ryan (R-Wis.) has publicly said he thinks Trump should release his taxes. But Ryan is not about to force Trump to do it. He's got enough disagreements with Trump to juggle, thank you very much.

Democrats aren't necessarily crushed that their bill doesn't have a chance in 2016. They seem quite content to use Republicans' reluctance to take it up as a politically convenient cudgel.

Take this weekend, when Wyden's bill got a shout-out from Senate Minority Leader Harry Reid (D-Nev.) after the New York Times reported that Trump suffered a loss so big in 1995, tax law would have let him avoid paying federal income taxes for the next 18 years. (An assertion the Trump team has not denied.)

Wyden and other Democratic leaders urged Republican leaders to bring this up for a vote this September, but it never came up for a vote.

Politically convenient or not, public opinion may be on Democrats' side: A September Washington Post-ABC News poll found a little under two-thirds of Americans think Trump is not justified in keeping his tax returns private. Back in August 2012, when GOP nominee Mitt Romney still hadn't released his tax returns, slightly less than half of respondents thought he should.

The tradition of releasing taxes started with President Richard Nixon, who released his tax returns in 1973 when his personal finances were under intense public scrutiny. Since then, presidents and major party presidential candidates have released their tax filings — some just one year's worth, others going back dozens of years.

Tax returns provide one of the clearest paper trails for how a candidate lives his or her financial life: how much they donate to charity, to whom they owe money, how much they made or lost in a year and whether any money is held offshore.

Public returns or no, candidates' financial lives aren't totally secret. They are required by law to share their sources of income and assets and liabilities, albeit in wide ranges that can leave a lot of room for interpretation. (Anything worth $50 million or more simply gets listed as $50 million+, a vagueness Trump's staff took advantage of when they released his personal financial disclosures last summer — and declared he was worth $10 billion: "This report was not designed for a man of Mr. Trump's massive wealth," the accompanying statement said.)

Congress passed the law requiring personal financial disclosures in 1978, after the Watergate scandal. And it was the last major piece of legislation dealing with presidential candidates' personal finances, according to Americans for Tax Fairness, which supports Wyden's bill.

Thanks in part to politics, that record is likely to still stand — even though 2016 looks to be the first time in four decades in which a major-party presidential nominee doesn't release his tax returns.