A House panel is preparing to accuse Rep. Maxine Waters of at least one ethics violation in her efforts to help a bank with ties to her husband, and the longtime Los Angeles Democrat plans to fight the charges in a House trial, according to a source familiar with the case.

The allegations were presented Friday to Waters, the source said, speaking on the condition of anonymity because the investigation is confidential.

Waters, an outspoken legislator who has held elective office in Sacramento or Washington for more than three decades, could not be reached Friday night.

The findings on the investigation into Waters by the Office of Congressional Ethics are expected to be made public on Monday.

That office, an independent watchdog created by Congress, referred the matter to the House Ethics Committee. The committee turned the matter over to a panel of two Democratic and two Republican lawmakers who have been conducting their own probe for months.

The allegations come as Rep. Charles B. Rangel (D-N.Y.) faces a House trial on 13 ethics allegations, adding to the political troubles of Democrats confronted with a tough battle to hold onto their House majority in the November midterm election. Rangel, 80, and Waters, 71, are both high-profile, longtime members of the Congressional Black Caucus.

One of Los Angeles’ most enduring black politicians, Waters came under scrutiny last year after Massachusetts-based OneUnited Bank, one of the nation’s largest minority-owned institutions, received $12 million in bailout funds.

The funding came three months after Waters, a senior member of the committee that oversees banking, helped arrange a meeting between officials of the bank, other minority-owned financial institutions and Treasury Department representatives.

Waters’ husband, Sidney Williams, had owned stock in the bank and served on its board.

Waters has previously said that she fully disclosed her husband’s ties to the bank.

She has said her efforts were consistent with her longtime work to promote opportunity for minority-owned businesses and lending in underserved communities, such as her South Central Los Angeles district.

As in Rangel’s case, a bipartisan panel of lawmakers will be formed to hear Waters’ case, probably in the fall, unless she and the committee reach a settlement.

Lawmakers in the past have accepted a reprimand to settle cases. Punishment can be as severe as censure and even expulsion from the House.

richard.simon@latimes.com

lisa.mascaro@latimes.com