Of course, a double-digit deficit will still be difficult to overcome. Critz is still an unknown to more than half the district — only 47 percent say they are familiar with the congressman — while Altmire is known by 71 percent of voters. More than half of voters view Altmire favorably, while 39 percent have a favorable view of Critz.

Critz's campaign points out that the two congressman remain essentially tied among voters who are aware of them both, and Critz carries a lower unfavorable rating — 8 percent — than Altmire, of whom nearly two in ten voters disapprove. Critz is also considered the favorite of unions in the region, who bring strong organization — and fundraising prowess — to his effort.

"It's going to be a lot of work, it's going to be a tough race, but this is not a good place for Jason Altmire to be in," Mikus said.

But Altmire's campaign could be difficult to slow, especially after earning the influential endorsement from the Allegheny County Democratic Party earlier this week.

"The defeat comes as a devastating blow to the Critz campaign, which in recent weeks had conducted an unprecedented level of voter outreach, including at least four expensive campaign mailings and three 'robocalls' from the candidate. The magnitude of Critz's defeat, especially given the resources spent by his campaign, raises questions as to his ability to break through to the more diverse electorate necessary to win the primary and general elections," Altmire's campaign said in a statement.

Critz downplayed Altmire's endorsement, arguing that he would continue to make inroads within the new district.

“We are very pleased to have done so well considering the fact that Jason Altmire has been working with the Committee six years, while we had just six weeks to campaign in Allegheny County," said Critz to the Pittsburgh Post-Gazette.