Trump is trying to block Congressional oversight at every turn including blocking tax returns from being released, blocking people from testifying, suing Congress, etc. He is trying to run out the clock. Congress can subpoena documents and people to testify and Trump plans to slow walk this in court. If a person fails to respond to a Congressional subpoena then Congress can ask the US Attorney of the District of Columbia to enforce this subpoena. However the Attorney General can overrule that person. With Barr working for Trump rather than for the constitution, we can definitely see him blocking the US Attorney of DC from enforcing that subpoena. So what can Congress do next? Is congress essentially impotent?
Congress can sue and bring this to court but that could take YEARS. Instead the Congress can use something called INHERENT CONTEMPT.
Inherent contempt
Under this process, the procedure for holding a person in contempt involves only the chamber concerned. Following a contempt citation, the person cited is arrested by the Sergeant-at-Arms for the House or Senate, brought to the floor of the chamber, held to answe charges by the presiding officer, and then subjected to punishment as the chamber may dictate (usually imprisonments for punishment reasons, imprisonment for coercive effect, or release from the contempt citation. Contempt of Congress Wikipedia.
The Congress CAN haul someone off to jail if he/she disobeys a Congressional subpoena.
Last time this was done in Congress was 1934 in the Senate. This was litigated all the way to SCOTUS and found to be constitutional in Jurney v MacCracken.
With a lawless President this may need to be the final step Congress may have to take to rain in this would-be DICKtator.