Mr. Papadopoulos filed his papers just two days after Ms. Hill, a freshman Democrat, said she would resign in the face of a House ethics investigation into allegations that she had a sexual relationship with a member of her congressional staff. She had been seen as a rising star in the caucus after defeating the Republican incumbent, Steve Knight, during the 2018 midterms, and her win helped put Democrats in control of the House.

Now her California district is back up for grabs. Republicans had controlled it since 1993, but voters there went for Hillary Clinton in 2016.

F.E.C. records show that seven Republicans — including Mr. Papadopoulos — and one Democrat have filed paperwork to run for the seat.

Christy Smith , a Democrat who is a California State Assemblywoman and longtime resident of Santa Clarita, has already announced that she will run to replace Ms. Hill in Congress. Latino Victory Fund, a progressive political action committee, has also said it “intends to recruit a strong and viable Latino candidate that will ensure this district remains Democratic for years to come.”

In 2017, Mr. Papadopoulos became the first aide in the Trump campaign to plead guilty in the special counsel’s investigation into ties between the president’s associates and the Russian government. He was sentenced last year to 14 days in jail for lying to the F.B.I. about his contacts with Russian intermediaries during the 2016 presidential race.