U.S. Congressman Eric Swalwell has dropped out of California's governor race following growing pressure over sexual misconduct allegations.

The Democrat has been accused by four women of misconduct ranging from sexual harassment to rape. He has denied the claims and vowed to defend himself with "facts".

Top allies had called him on him to exit the gubernatorial race in which he was projected as one of the frontrunners among Democratic candidates.

His departure comes at a pivotal moment in the wide-open primary race, just weeks before voters receive mail ballots ahead of the 2 June election to replace outgoing Governor Gavin Newsom and to lead the nation's most populous state.

In a post on X, he said: "I am suspending my campaign for Governor.

"To my family, staff, friends, and supporters, I am deeply sorry for the mistakes in judgment I've made in my past.

"I will fight the serious, false allegations that have been made - but that's my fight, not a campaign's."

Swalwell's decision to suspend his campaign comes two days after the first allegations emerged in the San Francisco Chronicle.

A former staff member told the newspaper that Swalwell first began making inappropriate comments, including soliciting her for sex and sending her sexual messages, shortly after she was hired to work in his district office in Castro Valley.

The woman, who has not been named, also said that in September 2019 she woke up naked in the congressman's hotel room with little recollection of what had happened the night before.

Five years later, when she was no longer on his staff, she met him for drinks at a gala. She said she had trouble remembering details of the night, but recalled pushing him away from her and telling him "no".

She added that she woke up with signs of sexual trauma on her body.

The woman's story was reportedly corroborated by text messages she sent friends at the time and by her former boyfriend, who told the newspaper that he had encouraged her to report the incident to the police.

Later on Friday, CNN published accusations from four women who worked for him, who had also made claims of sexual misconduct.

Within hours of the accusations surfacing, Swalwell lost backing from prominent supporters, including Senator Adam Schiff and House Democratic Leader Hakeem Jeffries.

Swalwell's legal team had sent cease-and-desist letters to two of the accusers the day prior, according to the broadcaster.

Swalwell vehemently denied the allegations in his statement on Friday.

"For nearly 20 years, I have served the public - as a prosecutor and a congressman and have always protected women. I will defend myself with the facts and where necessary bring legal action," he said.

"My focus in the coming days is to be with my wife and children and defend our decades of service against these lies."

At least 10 candidates are expected to be on the ballot in the non-partisan primary.

Republicans typically struggle to win statewide races in the heavily Democratic state, but a large group of Democrats have split the state's liberal voters, leaving two Republicans near the top of early primary polls.

The top two vote-getters in the primary move on to the 3 November general election, regardless of party preference or whether a candidate receives a majority of all votes cast in the primary.