Citigroup in March put restrictions on new retail business clients that sell guns to require that customers pass background checks after the deadly Parkland, Florida high school shooting, drawing the ire of the U.S. gun lobby.
"Recent news reports indicate that the Commissioner 'castigat[ed] them for straying into social policy' while 'glowering and speaking emphatically' about guns," the senators said in a letter.
"Most concerning are reports that Commissioner Piwowar delivered a 'thinly veiled threat' by suggesting that the Commission would withhold support for Citigroup’s regulatory request due to the company's unrelated and entirely lawful corporate practices relating to firearms."
The SEC did not immediately return a request for comment.
The letter was signed by Democratic Senators Chris Van Hollen, Kamala Harris, Christopher Murphy, Richard Blumenthal, Robert Menendez and Dianne Feinstein.
Weeks after Citigroup's decision, Bank of America said it would no longer lend to companies that make military-style firearms for civilians. The banks soon came under attack from Gun Owners of America and the National Rifle Association.
Piwowar made the "troubling remarks" to Citigroup executives during an April 24 meeting to discuss derivatives regulation, according to the letter, released by Van Hollen's office.
"While we are not advocating for or taking a position on the regulatory matter that was the subject of the meeting, we do have concerns that Commissioner Piwowar may have abused his government position in an attempt to unduly influence Citigroup to reverse a business decision that conflicts with his personal and political views," it said.
(Reporting by Doina Chiacu and Susan Heavey; Editing by Chizu Nomiyama and Jeffrey Benkoe)