On the same day, Oct. 4, that the House Intelligence Committee subpoenaed three employees of the opposition research firm Fusion GPS to testify in the Trump dossier investigation, the committee also subpoenaed TD Bank for Fusion's bank records.

Now, according to a source familiar with the situation, Fusion has asked a U.S. District Court in Washington, D.C., judge to prevent the bank from complying with the subpoena.

The move comes just days after two of those three Fusion employees asserted their Fifth Amendment right against self-incrimination rather than answer questions about the dossier. A third subpoenaed Fusion employee, founder Glenn Simpson, has not yet appeared before the House.

The committee's intention in sending the subpoena to TD Bank is to see if Fusion's bank records shed light on who financed the Trump dossier. That is one of the two most important questions in the dossier investigation -- the other being whether any U.S. intelligence or law enforcement agencies used the unverified dossier as a basis for surveillance applications to the Foreign Intelligence Surveillance Court.

Fusion has flatly refused to provide any information on its funding to either House or Senate investigators. The two officials refused to answer all questions from the House this week, and in an appearance before Senate Judiciary Committee investigators in August, Simpson also refused to answer the question.

It is not unprecedented for Congress to subpoena bank records. As a general rule, according to congressional investigators, banks usually comply without much fuss. But of course, this is not a routine case.