18 U.S. Code § 607 say :

"It shall be unlawful for an individual who is an officer or employee of the Federal Government, including the President, Vice President, and Members of Congress, to solicit or receive a donation of money or other thing of value in connection with a Federal, State, or local election, while in any room or building occupied in the discharge of official duties by an officer or employee of the United States, from any person."

He stopped to answer questions while walking along a hallway. It was not done while discharging his official duties, nor was it done in a room used to discharge official duties.

The law does not apply to him unless he get up on the senate floor and solicit donations.