A US House committee has formally charged a California Democrat with three counts of breaking the body's ethics rules.
The panel said Maxine Waters had used her influence to benefit a bank in which her husband had stock holdings.
Its report said her staff had improperly sought US treasury department aid to the bank during the 2008 financial crisis.
Ms Waters, from Los Angeles, has denied breaking House rules of conduct.
Lawyers for the 10-term Democrat also said that the committee had previously exonerated a congressman charged in a similar case, and said Ms Waters had disclosed her financial interest in the bank.
According to the charge sheet released by the House ethics committee on Monday, Ms Waters sought federal aid in September 2008 for a bank in which her husband held a substantial stock investment and had previously been a board member.
The alleged violations occurred in meetings, phone calls and e-mails during the banking crisis that engulfed the nation.
The bank, OneUnited, was on the brink of failure, which would have rendered Ms Waters' husband's investment worthless.
The committee said her staff had arranged a meeting between US treasury department officials and senior officials at the bank.
It said Ms Waters' chief-of-staff Mikael Moore, also her grandson, had helped the bank apply for and obtain federal aid under an October 2008 US bank bailout programme.
It said those actions had violated rules barring congressmen from using their influence for personal gain, and two other rules.
Ms Waters is the second senior Democrat in recent weeks to be charged with ethics violations, and analysts say it could hurt the Democrats' chances in November's mid-term elections.
Last month, Representative Charlie Rangel, a senior New York Democrat and powerful former chairman of the House Ways and Means committee, was charged with 13 ethical violations.
He has denied wrongdoing.