PITTSBURGH — U.S. Rep. Conor Lamb said Friday he is running for Pennsylvania’s open Senate seat, joining a crowded Democratic field in one of the nation’s most competitive races.

Lamb is seeking the nomination to replace outgoing GOP Sen. Pat Toomey. Lamb, a former Marine and federal prosecutor, rose to political prominence three years ago when he beat a Donald Trump-backed Republican in a special election that foreshadowed the 2018 Democratic takeover of the House.

The Senate race is wide open on both sides and is expected to be among the most expensive in next year’s U.S. midterm election. Toomey is retiring after two terms.

Lamb said in a YouTube video posted Friday that he is running, declaring: “I believe this is the most important Senate seat in the country.”

Lamb planned to launch his campaign Friday afternoon at a union hall in Pittsburgh.

Lamb faces a diverse lineup of Democratic candidates. They include the state’s lieutenant governor, John Fetterman; Philadelphia state House Rep. Malcolm Kenyatta, who is Black and openly gay; and anesthesiologist Val Arkoosh, a woman who chairs Montgomery County’s board of commissioners.