"We find it stunning and noteworthy that so far Mr. Lew has not provided adequate responses to congressional inquiries on the matter," the senators wrote to Obama on Tuesday.
"Congress needs a clearer understanding about his role in the violation of this law, including exactly when Mr. Lew first became aware of this legal requirement and what counsel, if any, he provided the administration on whether it should comply with this law."
The White House is legally required to send Congress a proposal to curb Medicare's spending whenever the program's trustees express concerns about its solvency in their annual report. Obama has not issued those proposals, though his annual budgets have included major Medicare savings.
Senate Republicans began targeting Lew, Obama's former chief of staff, on the issue earlier this month. His nomination could see a final floor vote on Wednesday.