VIRGINIA BEACH — Representative Scott Taylor, Republican of Virginia, has enjoyed a rapid political rise anchored in his valorous background as a member of the Navy SEALs, a credential with great resonance in a district that includes the world’s largest naval station and one of the highest concentrations of voters connected to the military. Fit, square-jawed and telegenic, he was expected to have a relatively easy time winning re-election in November.

But now he is threatened with a sudden fall: His campaign is facing accusations that it was part of an improper effort to help an independent candidate get on the ballot and siphon voters from his Democratic challenger. The allegations, which included using the names of dead people or voters who did not live in the district on signature petitions, were serious enough to warrant the appointment of a special prosecutor, and the independent candidate, Shaun Brown, was stricken from the ballot by the Virginia Supreme Court.

Mr. Taylor’s race is emblematic of an emerging problem for Republicans as they seek to maintain an increasingly tenuous grip on the House: A seat once considered relatively safe is now imperiled because of scandal, expanding an already broad field of Democratic opportunity.

Democrats, facing comparatively few legal problems and seeking to portray a “culture of corruption” under Republican leadership, have tried to lump Mr. Taylor in with about a half-dozen other Republican candidates whose campaigns have veered off course over accusations of misconduct. They represent a small fraction of the 23 House seats that Democrats need to reclaim control, but in a year when Democrats were already expected to make gains, their fates could prove crucial.