Republican presidential candidate Donald Trump and Democratic candidate Sen. Bernie Sanders (I-Vt.). (Associated Press file photos)

TWITTER EXPLODED Thursday with talk of a possible Donald Trump-Bernie Sanders debate before California’s June 7 primary. On at least one level, we think it would be appropriate for them to share a stage, since they also share an unacceptable lack of transparency.

Mr. Trump’s refusal to release his tax returns has been much discussed. Despite repeated promises to show his returns to voters, as every candidate of the modern era has done, he has found excuse after excuse to delay or deny the need to do so. The returns are under audit, he says — which does not prevent him from releasing what he submitted and swore was true. Tax returns do not show very much, he says — which makes us wonder why he is spending so much time and rhetorical effort keeping them hidden. Theories abound: He pays little or no tax; he does not have the income one would expect for someone who claims to be so successful; he gives little or nothing to charity.

Mr. Sanders’s stinginess with information has received less attention, but not because he has been much more forthcoming. The senator from Vermont has released a single year’s tax return — for 2014 — and only then after pressure from reporters. Before releasing his full 2014 return, he argued that his tax information is not particularly interesting. As with Mr. Trump, this has given rise to speculation about possible reasons for the delay and limited openness. Is Mr. Sanders, for example, attempting to avoid more scrutiny of his wife’s severance package from Burlington College? The liberal-arts school she headed subsequently was forced to close due to mismanagement and a dire lack of money.

Hillary Clinton long ago released eight years of tax returns. Jeb Bush released tax returns going back to 1981. Even Mitt Romney, who was reluctant to release his returns, eventually offered two years of tax information in 2012. Meanwhile, Mr. Sanders promised in April, “We will get out as much information as we can.” Clearly, he is not meeting his own standard.

Both Mr. Sanders and Mr. Trump are eroding an important norm in American political life — the expectation that voters should have a sense of presidential candidates’ financial conduct before they choose who should lead the country. As long as Mr. Sanders is a legitimate candidate, he does not deserve a pass.

Donald Trump's stance on presidential candidates has changed significantly over the years. Here's how. (Peter Stevenson/The Washington Post)

The fabric of norms, traditions and expectations that shape politicians’ behavior, stabilize the country’s political life and promote democratic accountability — unwritten rules of conduct, truthfulness and disclosure — are in greater danger now than at any other time in recent memory. As a result, some transparency rules may have to be written into law. Sen. Ron Wyden (D-Ore.) has proposed a bill that would require each party’s nominee to release three years’ worth of tax returns. Congress should consider the proposal, preferably requiring more years of disclosure and including primary candidates, too.

But why should it take legislation to force Mr. Trump or Mr. Sanders to do the right thing?