Four Democratic senators have sent a letter to White House adviser Steve Bannon asking that he clarify whether he is in violation of an ethics pledge when he reportedly communicated with Breitbart News staff.

The letter was headed by Massachusetts Sen. Elizabeth Warren and asked for "clarification on your recent communications" with Breitbart, which Bannon ran before taking on his role in the White House.

The letter referred to an anti-lobbying ethics pledge President Trump said he wanted all executive branch employees to sign, which includes language that commits signers not to participate in matters directly related to former employers.

During an interview on NBC's "Today" in mid-March, Alex Marlow, editor in chief of Breitbart, said that Bannon reaches out to him "every so often."

Another report in February said Bannon had been in touch with another Breitbart editor, Matthew Boyle, to complain about a story published on the site.

The Warren letter said Bannon's "communications with Breitbart appear to violate" the ethics pledge. It asked Bannon a series of questions, including whether he signed the White House pledge; if he received a waiver or authorization to communicate with Breitbart; and whether he was disciplined for any such communication.

A White House spokesperson has not returned a request Thursday for comment from the Washington Examiner.

Co-signing the letter were Democratic Sens. Sheldon Whitehouse of Rhode Island; Edward Markey of Massachusetts; and Mazie Hirono of Hawaii.