The White House suggested to BuzzFeed that there was nothing particularly unusual about friends of the president getting tours of the presidential military plane, and that members of the club who may have received invitations were likely to have been old friends of Trump.

Still, the Democratic senators said in their letter that the circumstances are different, given that Trump continues to own the properties in South Florida.

“Previous Presidents have provided friends and even campaign donors tours of Air Force One, but the reports that members of the President’s private club may have received such tours are particularly troubling because their relationship to the President arises out of an ongoing business relationship with the President,” the senators wrote.

“If true, these reports could mean that the President or his family have used government property for private gain — giving tours of Air Force One as a perk for club membership — or that some citizens have potentially been able to buy access to Air Force One via their personal payments to the President’s private club,” the letter continued.

Sens. Richard Blumenthal of Connecticut, Thomas R. Carper of Delaware and Sheldon Whitehouse of Rhode Island signed on to the Warren letter, in which they also raised readiness and defense concerns.