Mr. Critz of Johnstown the Democratic incumbent, has many of the same conservative positions as his Republican challenger from Sewickley, but Mr. Rothfus takes them a few clicks further right. Both criticize President Barack Obama’s health care bill, though the Democrat says he would keep parts and the Republican pushes for full repeal. Both are anti-abortion, but Mr. Critz supports exceptions in cases of rape, incest or the health of the mother while his GOP opponent supports it only when the mother’s health is at stake.

Their biggest differences are over taxes and trade.

A conservative intellectual in the mold of U.S. Sen. Pat Toomey, Mr. Rothfus is wont to quote historian Amity Shlaes when questioning government spending in a recession, and would consider cutting the Education Department and placing sunsets on other federal roles (though not defense, veteran or senior programs).

Mr. Critz supports raising taxes on the wealthy to help curb the nation’s deficit, while Mr. Rothfus says tax increases would dampen the very economic growth needed to reduce it.

“We can’t cut our way out of the current fiscal situation, so raising taxes on the very wealthiest — which at this point is at the lowest point since prior to Eisenhower being in office — I don’t see where that’s such an issue,” the Democrat said recently. “You cannot defend raising your expenses and decreasing your revenue, and think everything’s going to work out.”

The Democrat opposes trade deals with Central American and Asian countries, saying American manufacturers should be “given a fair playing field to compete.” The Republican supports them, saying “90 percent of the customers in this world are outside this country. … We have to continue to look for new markets.”

Keith Rothfus

The 12th District battle is the latest test of GOP growth in conservative Western Pennsylvania, and the weakening of the other side. Should Mr. Critz lose Nov. 6, the state likely would have one congressional Democrat west of Harrisburg — Mike Doyle, whose district contains all of Democratic-laden Pittsburgh, assuming he wins re-election.

Since his first run in 2010, Mr. Rothfus — a lawyer, former George W. Bush administration official and officer at a Christian law school — has presented himself as a private sector apostle, focused largely on economic issues and the debt.

Mr. Critz, a former aide to the late U.S. Rep. John Murtha, who also made his first congressional run that year, has cast himself as a pragmatic trustee for working families who supports some Obama administration policies but will fight others.

Both men — and local television viewers — are suffering through an advertising blitzkrieg as each side tries to redefine the other. Most of the ads go over the same old issues, largely related to Medicare and Mr. Obama’s policies, in the relentless attempt to sway the seniors and Obama critics prevalent in the new district.

Republican gains have made the 12th District one of the most competitive congressional races in the country.

Mr. Murtha’s old district was the only one nationwide that voted for Democrat John Kerry for president in 2004 but Republican John McCain in 2008. After the 2010 census, Republican map-makers in Harrisburg then stretched a district that was south and east of Pittsburgh up through the GOP-dominated North Hills to combine it with the 4th District seat held by U.S. Rep. Jason Altmire, D-McCandless, thereby boosting its Republican vote even further.

Even Democratic polling shows Mr. Obama to be unpopular across the new district, which stretches from southern Lawrence County into Beaver and through parts of Allegheny, Westmoreland, Somerset and Cambria counties.

When former President Bill Clinton, a high-profile and canny Obama supporter, stumped for Mr. Critz in Democratic-dominated Beaver recently, he never mentioned the president’s name.

The previous 4th District likewise voted for Mr. McCain in 2008, but it was still a shock when the unknown Mr. Rothfus beat former U.S. Attorney Mary Beth Buchanan in its GOP primary in 2010 and came within 2 percentage points of ousting Mr. Altmire that fall, despite being outspent 2-1.

That is different today, with the spending advantage flipped due to the rise of super-PAC spending. Outside groups poured $6.2 million into the race as of last week, which was the fourth most in any congressional race nationwide, according to a Los Angeles Times analysis. Attacks on the Democratic candidate accounted for $3 million — almost twice the $1.7 million attacking the Republican.

Mr. Rothfus spent $1.3 million through Sept. 30 of this year to $1.9 million by Mr. Critz, but the Democrat had to sink $1.4 million into his primary battle with Mr. Altmire. That was on top of $2.4 million he spent in the 2010 cycle to fend off self-funding GOP businessman Tim Burns, whose campaign put $2.2 million into the race.

Mr. Critz, 50, has basically been in a full sprint of fundraising, intraparty fights and repeated, drag-out election battles for three years, and has come out on top each time.

U.S. Rep. Mark Critz

“I feel like the Steelers in the fourth Super Bowl against the Rams,” Mr. Critz said last week, speaking of the 1979 season capper where the favored Pittsburgh squad trailed after three quarters but won. “When things looked down they didn’t get excited. They had a job to do.”

Mr. Critz leaned on heavy support from organized labor to help him beat the better-funded and -known Mr. Altmire this spring, and is again relying on his grass-roots abilities to help overcome the GOP funding advantage in the general election. While he commands support in his Cambria and Somerset home base, he also needs labor to help introduce him to Beaver County — which despite a nearly 2-1 Democratic registration edge also voted for Mr. McCain in 2008.

Mr. Rothfus has been campaigning there for three years and Mr. Critz 10 months.

“You don’t have to agree with everything that the administration at the federal level is doing, but let me tell you something — the other side is coming after you,” Mr. Critz told retired members of the Pennsylvania State Education Association, the state’s largest public sector union, during a luncheon last week. “It’s not just you. It’s the middle class, the working families. We’re under the thumb, and they’re trying to squash us.”

Westmoreland and Beaver counties have the highest per-capita participation in Medicare Advantage in the nation, and Mr. Critz has tried to use proposed Republican changes to Medicare for future retirees — including changing it partially to a voucher system — to bludgeon his opponent.

That is a common Democratic strategy this year and Mr. Rothfus has responded that some changes are required to keep the popular health coverage going, but regular Medicare will always be there as an option.

Like many others in the GOP — including vice presidential hopeful Rep. Paul Ryan of Wisconsin — Mr. Rothfus filmed an ad in which his mother-in-law scolds Mr. Critz over the charges.

Mr. Rothfus has no voting record (which is one reason the NRA endorsed Mr. Critz, who is a known entity to the firearms lobby, over his gun-supporting challenger). On the flip side, the Rothfus campaign continually hits its opponent for Mr. Obama’s health care bill even though it was approved in early 2010 before he entered Congress.

Mr. Critz has defended popular parts of the bill extending health benefits to young adults and those with pre-existing conditions, and its expanded drug coverage.

Mr. Rothfus was touring Beaver Falls specialty steel maker DB&S Corp. early this month when owner Larry Caracciolo stopped him in the snack room to complain about the high cost of his health care plan. It is set to increase by 19 percent next year for his 43 workers, costing him more than $52,000 — roughly the cost of adding a full-time worker to his lines.

“The health care thing is such a problem,” Mr. Caracciolo, a Republican, said. “You stay awake at night wondering, how am I going to make this happen?”

“Hold on. Hold on,” Mr. Rothfus told the company owner. “Help is on the way.”