A group of U.S. Senate Democrats is pressing large U.S. banks to scrap or significantly reduce overdraft and other fees they charge customers with insufficient funds.

In a letter sent to seven large firms Thursday, the group of five lawmakers -- including Senate Banking Chairman Sherrod Brown -- called for a "fairer and more transparent" fee structure.

Democratic lawmakers and regulators are placing heightened scrutiny on bank fees. The group cited recent research from the Consumer Financial Protection Bureau, which found nearly 80% of such fees are charged to only 9% of accounts. The CFPB is currently soliciting public feedback on ways to potentially curtail overdraft and other "junk fees," and a House Financial Services subcommittee will hold a hearing Thursday on the fees.

Under political and regulatory pressure, several large banks, including some that received letters, have taken steps to curtail such fees.

In February, Citigroup announced it would eliminate overdraft fees by this summer. JPMorgan Chase, Wells Fargo & Co and US Bancorp have moved to give customers extra time to bring their account balances above zero. Bank of America said it would reduce overdraft fees to $10 from $35 beginning in May and eliminate its "nonsufficient fund" fees.

JPMorgan had already taken other steps requested in the letter, such as eliminating nonsufficient fund fees, according to a spokeswoman.

But banking groups are resisting government efforts to eliminate overdraft fees, arguing they serve a useful purpose.

"A majority of consumers who use the product do so knowingly and count on it when unexpected expenses arise. As a result, contrary to what is stated in the letter, the Consumer Bankers Association believes taking action that would dramatically restrict overdraft could force many families out of the well-regulated, well-supervised banking and toward predatory payday lenders," said CBA spokesperson Lauren Bair Bianchi.

Copies of the letter were sent to chief executives at JPMorgan, Wells Fargo, Truist Financial Corp, PNC Financial Services Group, US Bancorp and Charles Schwab Corp.

Spokespeople for the other banks either declined to comment or did not respond to a request for comment.