National Republicans worry that Mr. Saccone, 59, is proving to be a lackluster candidate, an assessment that was reinforced when he raised only $200,000 as of the end of the year, nearly half of it in cash transfers from House lawmakers eager to preserve their majority.

To prop him up, the House Republican campaign arm hosted a fund-raiser Wednesday evening in Washington with Mr. Saccone; Representative Steve Stivers of Ohio, the chairman of the National Republican Congressional Committee; and each of the Republican winners of special House elections last year.

Mr. Saccone will return to Washington next month for a fund-raiser featuring the entire roster of House Republican leaders including Speaker Paul D. Ryan.

Further, a group of well-financed outside Republican groups is planning to unleash a biting advertising campaign against Conor Lamb, the Democratic nominee and a Marine Corps veteran. A pair of conservative organizations have already broadcast about $700,000 worth of commercials in the district and the best-funded House “super PAC,” the Congressional Leadership Fund, is preparing to go on the air next week with a spot blistering Mr. Lamb.

“We will attack Conor Lamb, we will define Conor Lamb, and we will explain why he is a Nancy Pelosi rubber stamp,” vowed Corry Bliss, who runs the Congressional Leadership Fund and had already opened two field offices in the district, referring to the House Democratic leader.

But Mr. Lamb, a former prosecutor, may not be so easy to link to Ms. Pelosi.

“They can throw anything they want at him, but he’s already said he’s not voting for Pelosi as speaker,” said Marcel L. Groen, the Pennsylvania Democratic chairman.