WASHINGTON — Senate Finance Committee staff members met this month with an Internal Revenue Service whistle-blower who has alleged that senior Treasury Department officials tried to exert influence over the mandatory audit of President Trump’s tax returns, a congressional aide said on Monday.

The whistle-blower contacted the staff of the House Ways and Means Committee over the summer and accused political appointees in the Treasury Department of improperly involving themselves in the audit and putting pressure of some kind on senior officials in the I.R.S.

The Ways and Means Committee has been reviewing the allegations, which were included in a complaint, and in early November the staffs of Senator Charles E. Grassley of Iowa, the Republican chairman of the Finance Committee, and Senator Ron Wyden of Oregon, the panel’s top Democrat, interviewed the I.R.S. employee.

The details of the allegations remain unclear, though a person familiar with the complaint has said that it did not directly implicate Treasury Secretary Steven Mnuchin in the political meddling.