U.S. Senate Democrats on Monday challenged the appointment of Matthew Whitaker as acting U.S. attorney general, arguing he is ineligible to lead the Justice Department in an interim capacity because he did not rise from a role that required Senate confirmation.

The lawsuit—filed by U.S. Sens. Richard Blumenthal, Sheldon Whitehouse and Mazie Hirono—seeks a court order preventing Whitaker from continuing to serve as the acting attorney general. The complaint, joining other legal challenges, is the first that squarely addresses Whitaker’s appointment.