Democratic presidential candidate U.S. Sen. Kirsten Gillibrand speaks during a rally in front of Trump International Hotel & Tower on March 24, 2019 in New York City.

Democratic presidential contender Sen. Kirsten Gillibrand's husband has for more than a decade held a financial stake in an obscure medical device company, even as the New York lawmaker voted in favor of legislation that expedited the approval process for such devices.

Gillibrand's husband Jonathan is an investor in WindCrest LLC, which in 2012 filed patent applications for its flagship invention, a tiny device designed to help guide wires in delicate surgeries. The company's business partners were still seeking to commercialize it as recently as this week, they told CNBC.

The couple, married nearly two decades ago, does not appear to have made any money from the stake in the company. Gillibrand has said in financial disclosures that the investment is worth less than $50,000. She has disclosed the stake every year she has been in office.

But what those disclosures don't reveal is that the money-losing venture could one day turn a profit, thanks to its invention. That payday, however, will require securing a deal with a device manufacturer, according to the Cleveland Clinic, which partnered with WindCrest on the invention.

The investment, though legal, raises questions about whether Gillibrand will profit from an industry she has played a role in regulating as a senator, and which she could influence as president.

And Gillibrand's reluctance to discuss it could weaken one of her chief arguments against President Donald Trump, whose opaque business dealings have earned scrutiny from Democrats and good government groups, and who has declined to release his tax returns.

Gillibrand has made financial transparency a key element of her campaign, and just this week made a splash by releasing her 2018 tax returns and calling on her 2020 rivals to do the same.

While Gillibrand's husband's stake in the WindCrest is mentioned in their joint tax filing released Wednesday, the senator does not appear to have publicly discussed the nature of its work. Gillibrand was the first declared presidential candidate to release last year's tax returns. When she did so, she urged her fellow Democratic contenders to do the same in the interest of transparency.

The senator's campaign declined to comment on the record for this story. An aide told CNBC that the investment was made by Jonathan Gillibrand in 2005, that the couple has not made a profit on it, and that Jonathan has only ever had a "small minority share in the company."

"To Jonathan's understanding, the investment is of little to no value," the aide said. "Senator Gillibrand was proud to release her tax returns first, and believes that radical transparency is important as an elected official."

Little has been reported about WindCrest, which claims to work "to advance the medical device industry and the care of patients" on its website. An email sent to the company bounced back, and a phone call went straight to the voicemail inbox of a separate company called Whittaker Solutions.

Allison Marshall Whittaker, listed as the owner of Whittaker Solutions and a member of WindCrest LLC on her LinkedIn page, declined to comment.

Whittaker provides "design, business, medical & equine info," according to its Delaware corporate charter, as well as "strategic analysis consulting services to businesses and individuals."