Sen. Bernie Sanders (I-Vt.) has launched an investigation into President Trump’s purge of numerous top federal labor officials, Axios has learned.

Why it matters: The probe, which Sanders opened this week, is one of the first signs of Democrats flexing oversight muscle against the new administration.

• Sanders requested from leaders at the National Labor Relations Board and Equal Employment Opportunity Commission any communications with the Trump transition team or administration, according to a letter obtained by Axios.
• That was after Trump this week took the unprecedented step of firing Democratic members on both boards — part of a string of actions testing presidential power.
• The firings have rendered both boards essentially inoperable, without enough members to carry out many of their responsibilities.

The big picture: Sanders, the top Democrat on the Senate Health, Education, Labor and Pensions Committee, called the firings “deeply concerning” and said some were “patently illegal.”

• While Trump’s firing of general counsels at the agencies is routine for new presidents, his termination of board members was highly unusual. Board members serve five-year terms, and can only be dismissed for cause.
• Sanders requested that the agencies provide all communications with the Trump camp related to the firings by Feb. 12.

Catch up quick: Senate Democrats last year failed to confirm a Democratic member of the NLRB, which would have guaranteed the party’s control over the board for the next two years.

• Axios has reached out to both the NLRB and EEOC for comment.