The special election to fill a vacant House seat is Tuesday, and polls show the race in a dead heat. A loss, or even a near-miss would be devastating to Republicans, since Trump carried the longtime GOP seat by about 20 points.

A Democratic pickup would portend major Democratic gains November. The district, in greater Pittsburgh, is exactly the kind of territory where blue-collar voters deserted Democrats in droves—and are now coming back.

Democrat Conor Lamb is a social conservative—he defends gun rights and opposes abortion. He makes the generational argument that Democrats need a new face as House minority leader. But on pocketbook issues he’s a progressive.

Lamb supports Trump’s tariff orders on steel and aluminum—and was calling for defense of American industry long before Trump was. Lamb has astutely directed his fire against House Speaker Paul Ryan, no friend of labor, since this election is not about Trump, but about which party will control the House.

This has partly bulletproofed him against right-wing attacks, but has not prevented his lackluster Republican opponent from trying to paint Lamb as a Pelosi clone in ads. Right-wing columnists, taking the opposite tack, are already spinning an anticipated loss by Republican state Representative Rick Saccone as the result of Lamb not being a true Democrat.

This is of course nonsense. He’s had resounding support from the labor movement, and is a longtime friend of unions, unlike his Republican opponent, who supports union-busting. The lackluster Saccone is another element of Republican spin.

If Lamb wins, or even comes close, this is a calamity for Republicans—and a sign that the blue wave is still building. In some regions of the country, Democrats will need to make their peace with social conservatives. Progressives will still dominate a new Democratic House.