In a letter to the Treasury secretary, the Democratic senator warned against meddling with a system that processes trillions of dollars of transactions.

Robert F. Kennedy Jr. also disclosed that he had reached at least one settlement agreement with a company or individual that has accused him of “misconduct or inappropriate behavior.”

Senators had objected to the financial conflict posed by Robert F. Kennedy Jr.’s decision to retain a financial stake in lawsuits against a major drug company. It remains unclear whether any proceeds would be redirected.

The settlement’s existence after a 2017 accusation had been documented before the defense secretary nominee’s confirmation hearing this month, but the amount had not been known publicly.

President-elect Donald J. Trump’s pick for defense secretary had a more amicable exchange with Joni Ernst, a Republican, who is considered a key vote.