Scarcely three months after he was expelled from the House of Representatives and bitterly swore his intent never to return, George Santos went back on his word and declared his intent to run again.

Mr. Santos made the announcement on Thursday evening after returning to Washington, where he made use of the lifetime floor privileges given to members of the House and attended the State of the Union address. And as he had done throughout his brief tenure on Capitol Hill, he attempted to seize the spotlight, posting the news on social media in the middle of President Biden’s speech.

The disgraced former representative, who was expelled from office after an overwhelming bipartisan vote and faces 23 felony counts in an unresolved criminal case, made his announcement on Thursday evening on social media. He said the decision had followed “a lot of prayer” and consultation with friends and family members.

Mr. Santos will not run for his old seat, in Long Island and Queens, for which Tom Suozzi, a Democrat, recently won a special election. Instead, Mr. Santos said that he would attempt to unseat Representative Nick LaLota in New York’s First Congressional District, on Long Island’s eastern end.

Mr. Santos filed paperwork with the Federal Election Commission on Thursday designating a campaign committee and declaring his intent to challenge Mr. LaLota in the Republican primary in June.