Democrats have no authority to conduct oversight if the Trump administration regardless of what the law says:
The Department of Homeland Security has imposed new limits on visits by members of Congress and their staff to immigration enforcement facilities, intensifying a conflict between federal immigration officials and Democratic lawmakers over the separation of powers.
Under federal law, members of Congress can make unannounced oversight visits to immigration facilities that “detain or otherwise house aliens.” Lawmakers are not required to provide “prior notice of the intent to enter a facility” to conduct oversight, though members of their staff must request a visit at least 24 hours in advance.
But in guidance released this month, Immigration and Customs Enforcement asks members of Congress to give at least 72 hours notice for a visit to its facilities. Asked about the policy, a spokeswoman for the Department of Homeland Security, which oversees ICE, went even further, suggesting that federal officials would not be allowed entry unless they provided a week’s notice.
They are just making up new regulations on the fly. I guess the congressional Democrats can sue the administration. I’m sure it will all be worked out by 2029.