Congressional oversight is one of the implied powers required for congress to carry out their job as expressed in the constitution.
There are numerous statutes that cover congressional oversight.
There's debate over whether a president can be arrested, but it's clear that the house is given explicit authority to impeach the president and the senate to remove.
Saying they need to take it to the court for them to decide is a red herring of sorts. It's also suggesting that congress abdicate their duty to appear more fair.
Anyway you can read up on some those statutes. I believe Wikipedia has links to them
Congressional oversight - Wikipedia