The lawmakers said they were concerned by reports that Trump was still using an old Android device for his frequent personal Twitter messages.
"Did Trump receive a secured, encrypted smartphone for his personal use on or before Jan. 20? If so, is he using it?," said a tweet Tuesday by Senator Tom Carper, who along with fellow Democrat Claire McCaskill released a letter to the administration requesting information on the president's device.
"Trump should be well aware by now of the appropriate and necessary protocol to safeguard our nation's secrets."
The letter from the two lawmakers, dated February 9, was sent to Defense Secretary James Mattis along with Homeland Security chief John Kelly and the National Security Agency director Michael Rogers. The senators released the letter late Monday.
The lawmakers said they were concerned by reports that Trump was still using an Android device that may be several years old for his frequent personal Twitter messages.
"While it is important for the president to have the ability to communicate electronically, it is equally important that he does so in a manner that is secure and that ensures the preservation of presidential records," the letter said.
"The national security risks of compromising a smartphone used by a senior government official, such as the president of the United States, are considerable."
The New York Times reported last month that while Trump had received a new, secure device after his inauguration, he still relied on his older device despite protests from aides.
That report prompted a flurry of comments from security experts who argued that the president would be inviting danger by using his old personal phone.
Trump's smartphone "would probably be the most widely prized device on the internet for hackers -- and top of the target list for intelligence agencies around the world," said independent security researcher Graham Cluley in a blog post Tuesday.
Last month, Nicholas Weaver of the International Computer Science Institute in Berkeley, California, warned that "Trump's continued use of a dangerously insecure, out-of-date Android device should cause real panic."
Writing on the Lawfare blog, Weaver noted that hackers could gain access to the phone's location as well as its microphone and camera and that "the working assumption should be that Trump's phone is compromised by at least one -- probably multiple -- hostile foreign intelligence services and is actively being exploited."