Rep. Mick Mulvaney, the staunch conservative nominated to become President-elect Donald Trump’s budget chief, failed to pay more than $15,000 in federal payroll taxes for a past household employee, he told the Senate Budget Committee in a questionnaire.

“I have come to learn, during the confirmation review process, that I failed to pay FICA and federal and state unemployment taxes on a household employee for the years 2000-2004,” Mulvaney, R-S.C., wrote in a section of the document, obtained by Roll Call on Wednesday. “Upon discovery of that shortfall, I paid the federal taxes.”

Mulvaney will likely face tough questions from Senate Democrats about the revelation when he appears before the Budget panel and the Homeland Security and Governmental Affairs Committee on Jan. 24. Breaches of tax law have tripped up or even derailed other presidents’ nominees for Cabinet and other administration posts.

The tea party conservative, a leading deficit hawk in Congress, said the amount he owed and has since paid in federal Social Security, Medicare and unemployment taxes was $15,583, not including penalties and interest, which have not been determined. He said the amount owed in state taxes has also not been determined yet.

In a separate question that asked if he had ever to his knowledge employed an individual not authorized to work in the United States, Mulvaney answered “No.”

The document obtained by Roll Call does not include the name of the employee for whom taxes are owed or specify what his or her job was.

The four-term congressman was tapped by Trump in mid-December to serve as director of the Office of Management and Budget, a key post that will have a hand in every major item on Trump’s agenda.

Senate Democrats quickly pounced on the news of Mulvaney’s back taxes, which was first reported by The New York Times.

“When other previous cabinet nominees failed to pay their fair share in taxes, Senate Republicans forced those nominees to withdraw from consideration,” said Democratic Leader Charles E. Schumer of New York in a statement. “If failure to pay taxes was disqualifying for Democratic nominees, then the same should be true for Republican nominees.”

Schumer, speaking to reporters, pressed his point that Mulvaney should not become OMB director if such revelations torpedoed Democratic nominees. “What’s good for the goose is good for the gander,” said Schumer.

Sen. Ron Wyden, D-Ore., called the reports “very troubling.”

Some of President Barack Obama’s nominees faced similar scrutiny in 2009 over lapses in tax compliance. Former Senate Majority Leader Tom Daschle, D-S.D., withdrew his nomination for Health and Human Services secretary in February 2009 after disclosing that he had paid about $140,000 in back taxes.

Earlier that year, Obama’s pick for Treasury secretary, Timothy F. Geithner, also disclosed tax problems; he was confirmed by the Senate, but 34 senators voted against his confirmation.

In 1993, President Bill Clinton’s first two nominations for attorney general were unsuccessful after it was discovered both had employed undocumented immigrants as nannies.

The Trump transition team and Mulvaney’s congressional office did not respond to requests for comment.

A spokesman for Senate Budget Chairman Michael B. Enzi, R-Wyo., declined to comment.

Alan K. Ota and Kellie Mejdrich contributed to this report.