Del. Bill Janis, the Republican whip in the House of Delegates, officially announced at a news conference Wednesday that he would not seek re-election in November.

Instead, Janis will run as an independent for Henrico County Commonwealth's Attorney, taking on the embattled Republican candidate Matthew Geary, who party leaders abandoned after he admitted to an extramarital affair.

“The person who sits in the chair of the commonwealth’s attorney must be serious,’’ Janis said. “He must be trustworthy. There can be absolutely no question about his integrity. Or his character. Or his sound judgment.”

Janis, elected in 2001 to a district that includes parts of Henrico, Louisa and Goochland counties, took a lead role in judicial appointments and is the sponsor of the congressional redistricting map.

Both parties have until Tuesday to field a candidate in the Republican-leaning district for the November election.

Steve Thomas, chairman of the Spotsylvania County Republican Committee who works at a consulting firm, is expected to run.

House Minority Leader Ward L. Armstrong (D-Henry) acknowledged that the district is Republican-leaning, but said Democrats were still trying to find a candidate.

“It’s a tough slog,’’ he said. “We have a tough slog in a lot of races.”