“If he wins, you’re going to see probably another half a dozen Republicans say they’re not running again,” Mr. Biden told a reporter on a meet-and-greet rope line, adding that Mr. Lamb could also show the way toward that holiest of Democratic grails: “getting back working-class people supporting us again.”…

If Mr. Lamb, a Marine veteran and a former federal prosecutor, defeats State Representative Rick Saccone, a Republican, in a district that the president carried by 20 points, the alarm bells will be audible across the country. The Democrats will have shown that they can breach the Republican hold on the blue-collar white voters who helped deliver Mr. Trump the presidency.

“If the Democrats were to prevail in western Pennsylvania, that would be quite an earthquake, let’s face it,” said Representative Charlie Dent, a Republican who represents a district in the eastern part of the state. “If a strong pro-Trump district like this goes the other way, it would send a bad signal around the country in districts far more competitive than this one.”