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"It is with a broken heart that today I announce my resignation from Congress," she wrote in a statement released Sunday.

"Having private photos of personal moments weaponised against me has been an appalling invasion of my privacy. It's also illegal, and we are currently pursuing all of our available legal options," she added. "However, I know that as long as I am in Congress, we'll live fearful of what might come next and how much it will hurt."

Hill's statement provided no details on that or when she would step down. Hill's office and campaign provided no additional public comment.

Her abrupt fall came after a blazing rise in which she won the last Republican- held House seat anchored in Los Angeles County, part of a rout that saw GOP House members driven out of their seats in Southern California.

Hill was elected by 9 percentage points last year, ousting two-term Republican Stephen Knight and capturing the district for her party for the first time since 1990. Democratic presidential candidate Hillary Clinton carried the district in 2016 by 7 points.

Hill's campaign had raised a healthy $US2.2 million so far this year, putting her on track for a strong re-election bid.

Citing the more than 4-in-10 district voters from minority groups, Cheri Bustos, who heads House Democrats' campaign committee, said there was "no doubt" her party would hold the district next year.

But Tom Emmer, chairman of the House GOP campaign arm, said Republicans "look forward to winning back this seat".

Nationally, Hill was part of the wave that flipped the House to Democratic control. House Speaker Nancy Pelosi picked her for a coveted leadership seat. Hill also is vice-chair of the powerful House Oversight Committee.

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In a statement Sunday, Pelosi praised Hill for bringing "a powerful commitment to her community and a bright vision for the future" to the House but added: "She has acknowledged errors in judgment that made her continued service as a member untenable. We must ensure a climate of integrity and dignity in the Congress, and in all workplaces."

In recent days, Hill's Los Angeles-area constituents were wrestling with questions about who might have been wronged, how much it matters and whether any punishment was warranted.

Hill, celebrated as a face of millennial change in Congress, said in her statement that she had no choice but to step down.

"This is what needs to happen so that the good people who supported me will no longer be subjected to the pain inflicted by my abusive husband and the brutality of hateful political operatives who seem to happily provide a platform to a monster who is driving a smear campaign built around cyber exploitation," Hill said. "I can no longer allow my community, family, friends, staff, supporters, and especially the children who look up to me as a role model, to suffer this unprecedented brand of cruelty."

She apologised to "people who have been hurt".

She's asked US Capitol Police to investigate potential legal violations for posting and distributing the photos online without her consent.

Hill is divorcing her husband, Kenneth Heslep, and says he is abusive. While not providing any evidence of abuse, Hill says she turned elsewhere for companionship because of their turbulent relationship and lamented that "the deeply personal matter of my divorce has been brought into public view".

In court papers, Heslep sketches a different story, one in which he was rejected by an ambitious wife after agreeing to her request that he play the role of house-husband, while she pursued her career aspirations. He said she left him soon after being elected to Congress.

"Our agreement was that I would stay at home and take care of all the domestic duties and responsibilities while [Hill] worked," Heslep said in documents filed in July in Los Angeles Superior Court. He did not allege any extramarital affairs by his wife.