JOHNSTOWN — Voters on Mark Critz’s home turf picked up the first-term congressman and hauled him to an improbable victory in Tuesday’s Democratic primary election.

With all of the 12th Congressional District’s 631 precincts reporting, Critz had garnered 51 percent of the vote compared with 49 percent for his rival, U.S. Rep. Jason Altmire.

But the Johnstown-based Critz grabbed 91 percent of the vote in Cambria County and 87 percent in Somerset County, allowing him to push past Altmire’s numerical advantage in western parts of the district.

“You have no idea how overwhelming this is to me,” Critz told a ballroom packed with cheering supporters at downtown Johnstown’s Holiday Inn.

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Altmire, a three-term congressman based in Pittsburgh’s North Hills, cited what he called Critz’s “astounding” Cambria County numbers in a prepared statement.

“It was that turnout that won the election, and I have no doubt that the remainder of the district will have the same level of support for Congressman Critz as they get to know him,” Altmire said.

Altmire, who had gained 69 percent of the vote in Allegheny and Beaver counties, pledged his “full support” for Critz in the fall campaign. Critz will face Allegheny County Republican Keith Rothfus, who had no opposition in the Republican primary.

Rothfus late Tuesday issued a statement saying he anticipated “a spirited discussion in the coming months regarding the direction this country is headed.”

Critz spoke in the same hotel where he delivered two victory speeches in 2010, when he took over the 12th district seat that had been held for 36 years by his former boss, the late John Murtha.

He thanked his staff and union supporters, saying organized labor “stepped up and put people on the ground.”

“Everywhere I went, I saw the Steelworkers with their shirts on,” Critz said.

“A lot of people made this happen,” he added. “It was so many people working together and coordinating.”

The two lawmakers were pulled into a redrawn 12th district last year during the state’s Republican-controlled redistricting process.

And the campaign had made political combatants out of two friends with a similar tendency toward socially conservative stances. In two largely noneventful debates and in interviews, both had talked about their anti-abortion beliefs, their support for the coal and natural-gas industries and their commitment to preserving Social Security and Medicare.

There were a few disagreements. For instance, Altmire went after Critz for voting to cut federal funding for Planned Parenthood, claiming that Critz did not support women’s-health initiatives – an assertion the Johnstown lawmaker denied.

Critz, meanwhile, hit Altmire for voting for a Republican-backed balanced-budget amendment. Critz claimed the move would have gutted Social Security and Medicare, while Altmire said that argument relied on flawed data.

Mostly, though, the two men spent a lot of time talking about their supporters.

Altmire had contended the math was simple: About 66 percent of the population in the new 12th district are his current constituents in the 4th district.

Altmire also pointed to overwhelming support from Democratic committees in Allegheny, Beaver and Westmoreland counties. Critz won Democratic endorsement only in Cambria County; the two other counties in the district – Lawrence and Somerset – did not endorse.

But Critz received backing from every major union including the United Steelworkers, United Mine Workers and the Service Employees International Union. In the campaign’s final weeks, Critz repeatedly said he believed the AFL-CIO’s “ground game” could push him over the top.

The two campaigns each had released two internal polls, and all four surveys showed Altmire ahead by margins as large as 24 percentage points and as small as 7 points.

An independent poll last week appeared to show Critz narrowing the gap, since he was behind Altmire by only 4 points.

At the time, Critz’s campaign pointed out that the final poll had been conducted after the congressman had been endorsed by former President Bill Clinton.