“To put it bluntly, these payment systems simply cannot fail, and any politically motivated meddling in them risks severe damage to our country and the economy,” Senator Ron Wyden of Oregon, the top Democrat on the Senate Finance Committee, wrote in a letter to Mr. Bessent on Friday. “I can think of no good reason why political operators who have demonstrated a blatant disregard for the law would need access to these sensitive, mission-critical systems.”

Mr. Wyden followed up on Saturday to express concern that access to the payment system had already been granted and pointed to Mr. Musk’s potential conflicts of interest.

“Social Security and Medicare benefits, grants, payments to government contractors, including those that compete directly with Musk’s own companies. All of it,” he wrote on social media.