Reflecting the high political stakes, outside groups on both sides have flooded local airwaves with advertising. And each party has called in its superstars to energize voters.
Trump was expected to visit the area Saturday on behalf of the Republican candidate, state Rep. Rick Saccone.
There have been several elections since Trump's 2016 victory, but none has tested the loyalty of the white working-class voters who fueled his winning coalition across the industrial Midwest more than this one.
The western Pennsylvania district, which stretches from suburban Pittsburgh to the West Virginia border, is overwhelmingly white and features an estimated 17,000 steelworkers.
"We started as a divided fragmented group," Darrin Kelly, president of the Allegheny County Labor Council, said before Biden and Lamb took the stage. "Today, as we come to the finish line, we are a united unstoppable army."
A native son of working-class Scranton, Pennsylvania, Biden worked to ensure that the union "army" would unite behind Lamb and the Democratic Party once again.
"He's not afraid to say the word 'union,'" Biden said of Lamb at a subsequent appearance at a suburban Pittsburgh university. He told the carpenters: "It makes me angry when we're not respected — when you're not respected."
Lamb, too, offered warm words for his would-be labor allies in next week's contest.
"You are the heart and soul of this campaign," he said.