Only members of congressional committees have broad access to the country’s secretive spy agencies, allowing them to ensure that they operate efficiently, effectively and legally.

But this week, the Pentagon canceled a classified visit to a military spy agency by the top Democrat on the Senate Intelligence Committee, after the far-right conspiracy theorist Laura Loomer complained.

The Trump administration has been steadily eroding the power of Congress, chipping away at its ability to set spending priorities and ignoring its role in authorizing military force. Restrictions on oversight now appear to be an emerging front in the administration’s efforts to enhance executive authority.

Senator Mark Warner of Virginia, the top Democrat on the Intelligence Committee and a long-serving member of the panel, had planned to visit the Virginia headquarters of the National Geospatial-Intelligence Agency this week. But Pentagon officials told his office on Tuesday that the visit was canceled, and also imposed new requirements on congressional visits to military intelligence facilities.

The Trump administration has blocked access to Immigration and Customs Enforcement facilities despite a law that allows members of Congress to make unannounced visits. Congressional Democrats sued to gain entrance to them.

But Mr. Warner’s visit was different, with the purpose of conducting oversight of an agency out of the spotlight and removed from the divisive political battles over immigration. And while the public and journalists have broad access to most government agencies, intelligence agencies operate under a veil of secrecy, making congressional oversight particularly important.