After three years of Democrats attacking President Trump for refusing to release his tax returns, it might seem natural for the Democratic presidential hopefuls to release their own returns as quickly and fully as possible.

But while transparency has become a rallying cry for the party and its base, many of the Democrats vying to challenge Trump in 2020 haven’t been quick to disclose their own personal tax returns, saying it will take some time for the documents to be assembled or offering vague explanations for the delay.

Sen. Kirsten Gillibrand of New York publicly released her 2018 tax returns on Wednesday, becoming the first Democratic contender to do so and challenging her rivals to take the same step, as she seeks to distinguish herself in a crowded Democratic field.

According to a tax return posted on her campaign website, Gillibrand reported earning about $214,000 in adjusted gross income, including $167,634 from her Senate salary and another $50,000 from a book deal. She paid $29,170 in federal taxes.

“This is what transparency and accountability is all about,” Gillibrand said in a short video posted to her YouTube account, adding that she wanted voters to “know I am beholden to no one.” The two-term senator, who has sometimes struggled to attract attention to her campaign, urged voters to join her “in calling on every presidential candidate to disclose their taxes.”

Trump’s decision in 2016 not to release his tax returns, as presidential candidates had done since the 1970s, symbolized for many Democrats his lack of transparency and the unanswered questions about his sprawling business empire. For Trump’s supporters, it illustrated his willingness to violate political norms and resist pressure from detractors.

The Democratic candidates now find themselves in a potentially challenging spot. They face pressure to release years’ worth their returns, to emphasize to energized primary voters how different they are from Trump and how they reject his approach to government service. At the same time, tax returns can sometimes include embarrassing information or disclose data at odds with a candidate’s political self-portrait — such as a lofty income that might not sit well with Americans living paycheck to paycheck.

Democrats are wrestling not only with when to release their taxes, but for how many years back, in what is a purely voluntary exercise. Trump, after all, established that a candidate can win the presidency without releasing his returns, a proposition no one had tested for decades.

Trump has repeatedly referred to an ongoing audit by the Internal Revenue Service to explain the secrecy about his personal taxes. Some watchdogs worry candidates will now cite Trump for their own lack of disclosure.

“There’s been decades of people doing things a certain way, disclosing their taxes and divesting from their assets because that’s the way things were done,” said Jordan Libowitz of Citizens for Responsibility and Ethics in Washington (CREW), an ethics watchdog. “But now there is a fear that people will say, ‘Trump didn’t do it, so we’re not going to do it.’ ”

The group recently urged all presidential hopefuls, regardless of party, to commit to releasing 10 years’ worth of tax returns — arguing it would be a good standard not just for public officials, who are often required to disclose some information about their finances, but for political outsiders like Trump, whose finances may not be as transparent.

Democrats have offered mixed signals about the standards they would like to see applied and which ones they intend to follow.

Other than Gillibrand, only Sen. Elizabeth Warren of Massachusetts has offered an extensive look at her tax records — posting filings on her campaign site dating back to 2008, when the then-Harvard Law professor was appointed to a congressional panel overseeing the government’s response to the 2008 financial crisis. Warren, who prepares her own taxes, has yet to file her 2018 return but plans to make it public when she does, according to her campaign.

Warren frequently touts her tax transparency on the campaign trail and believes 2020 presidential candidates should be required to release eight years of tax returns, a campaign spokeswoman said. She has introduced legislation that would require the IRS to release eight years of returns for all candidates for president and vice president.

In Nashua, N.H., last month at a town hall meeting, Warren earned raucous applause by saying, “Anyone who wants to run for federal office should have to put their tax returns online.”

Other Democratic hopefuls have offered sometimes-vague answers about whether and when they plan to make their tax returns public, and what they would like to see their fellow Democrats do.

After he was criticized for releasing a single year of returns during his 2016 presidential bid, Sen. Bernie Sanders of Vermont said at a CNN town hall last month he would release “sooner rather than later” a decade’s worth of the documents. Asked for an update on Wednesday, his campaign manager Faiz Shakir replied, “Nothing for the record at this time.”

Sens. Kamala D. Harris of California and Amy Klobuchar of Minnesota have also said they will release their returns, though they have not offered details on how many years they might release or when. Likewise, former Texas congressman Beto O’Rourke told voters last week that he planned to release his taxes but offered no further details, and his campaign did not respond to a request for comment.

Sen. Cory Booker of New Jersey has offered reporters limited access to his tax filings in past elections but has not said if he will follow Warren and Gillibrand in releasing a more complete look now that he is seeking the presidency.

Former Vice President Joe Biden, who is expected to announce his decision on jumping in the race in coming weeks, has released 18 years of tax returns, covering 1998 to 2015 — beginning 10 years before he sought the White House on a ticket with Barack Obama. His spokesman did not respond to a request for comment about future disclosures.

While making tax returns public is a voluntary act, all candidates have to offer some details about their finances in a personal financial disclosure report, which must be filed with the Federal Election Commission and the Office of Government Ethics by May 15.

In some cases, PFDs, as they are known, can be more illuminating than tax returns, offering details on speaking fees and the value of certain assets.

In other cases, a tax return provides more information. Some Democrats privately worry about an appearance problem if some 2020 hopefuls hold back on releasing their tax returns after the party attacked Trump so forcefully for doing so.

Former Maryland governor Martin O’Malley, who ran for president in 2016 and released five years of his taxes at that time, said transparency is a key element for this year’s Democratic hopefuls, as they seek to assure voters they can restore trust, transparency and ethics to the White House following a president who Democrats believe has abandoned them.

“That’s especially true, I think, of their own personal finances,” said O’Malley, who has endorsed O’Rourke. “Especially with what we’re going through with Donald Trump.”

Annie Linskey, Sean Sullivan and Chelsea Janes contributed to this report.