Michigan billionaire Betsy DeVos is scheduled to appear Tuesday before the Senate Committee on Health, Education, Labor and Pensions. (Jimlo Scalzo/EPA)

A key Senate Democrat has asked Betsy DeVos, President-elect Donald Trump’s pick for education secretary, to turn over three years of tax returns to the committee overseeing her confirmation process.

Sen. Patty Murray (D-Wash.), ranking member of the committee on Health, Education, Pensions and Labor (HELP), made the request in a letter dated Jan. 13, days after Murray voiced concern about DeVos’s “extensive financial entanglements and potential conflicts of interest.”

While the HELP committee has not required education nominees to share tax returns in the past, Murray wrote, several have done so voluntarily, including two George W. Bush nominees, Rod Paige and Margaret Spellings.

Murray said she plans to try to revise committee rules to require all presidential nominees to share tax returns. In the meantime, she said, DeVos should voluntarily disclose her tax returns “to ensure that the Committee has an opportunity to fully understand the challenges your investments and other financial transactions may present to your coming nomination as Secretary of Education.”

[Betsy DeVos omitted $125,000 anti-union donation from her Senate disclosure form]

A spokeswoman for Trump’s transition team did not respond to a request for comment.

DeVos, a Michigan billionaire who has lobbied forcefully for charter schools and taxpayer-funded vouchers, is one of several Trump nominees that Democrats have singled out for extra scrutiny.

Her personal investments and significant political donations to GOP candidates and conservative causes are likely to come up during her confirmation hearing, which is scheduled to begin at 5 p.m. Tuesday. The hearing, originally scheduled for Jan. 11, was postponed after Murray and other Democrats argued for a delay given that the Office of Government Ethics — which is responsible for identifying and resolving conflicts of interest — had not yet finished its ethics review of DeVos.

That ethics review was still not complete as of Jan. 16, according to Democratic and Republican Senate aides.

Sen. Lamar Alexander (R-Tenn.), chairman of the HELP committee, has said he has no concerns about DeVos’s paperwork and believes she will be an excellent education secretary.