PITTSBURGH — After a Tuesday rally for Pennsylvania Democrat Conor Lamb at the Carpenters Training Center here, Preston Sparks stood outside, holding a clipboard as he filled out a form.

But he wasn’t signing up with the House candidate’s campaign. Instead he’s volunteering with local unions, who have launched a coordinated ground game to support Lamb ahead of the March 13 special election in the 18th District.

“Somebody’s got to help,” said Sparks, a 62-year-old carpenter from Bear Rocks.

Roughly two dozen unions are mobilizing workers like Sparks in the final days of the race to help Lamb do the improbable — win.

A Democrat has not represented the area since 2002, after districts were redrawn. And Republican nominees have carried the district by double digits in the past three presidential elections.