A key part of the answer is the way the district's boundaries changed after the 2010 census, according to Kyle Kondik. He's a Cleveland native who follows Rust Belt politics and is the managing editor of Sabato's Crystal Ball, a website that offers analysis on elections across the country through the University of Virginia Center for Politics.
The 12th was redrawn to absorb most of the 4th Congressional District, which was previously occupied by center-right Democrat Jason Altmire, and is more conservative than the 12th at the presidential level.
In 2008, for example, 49.8 percent of the 12th District went to Obama. But if the 12th District had absorbed the 4th back in 2008, only 45.2 percent of the district would have voted for the President.
"They redrew the 12th in a way that it would be won by Republicans — that was the goal of the Republicans in Harrisburg, and that was successful," Kondik says.
The new district includes all of Beaver County and parts of Allegheny, Cambria, Lawrence, Somerset and Westmoreland counties — extending from Ellwood City in the north, south into northern Allegheny County and all the way to Johnstown in the east.
"Democrats really didn't hold that seat; it's a new seat," says Don Friedman, a Democratic consultant. "Their goal was to give [Democratic Congressman] Mike Doyle as many Democrats as they could," since his district already covers liberal strongholds in Pittsburgh.
But redistricting only tells part of the story: In 2012, Democratic incumbent Mark Critz, another moderate, lost to Rothfus by just 3 percentage-points — after the lines were redrawn. And in the old 4th District (the more conservative area that was largely pushed into the 12th), in 2010, Democratic incumbent Jason Altmire fended off a challenge from Rothfus, narrowly beating him by 1.6 percentage points.
The 12th District "should never be written off," argues Mike Mikus, a former Altmire and Critz campaign manager. "It got a little tougher, but in the right year, this seat can be won."
Mikus, who now works for Democratic gubernatorial candidate Tom Wolf's Fresh Start PAC, says the top of the ticket might dictate how competitive the district is. In an off-year election with an unpopular president, it will be an uphill battle for Democrats, Mikus says.
"Part of it is that the national Democratic Party brand is just in really terrible shape in Appalachia," agrees the University of Virginia's Kondik, who says 52 out of 62 U.S. House seats that contain at least one Appalachian county are held by Republicans.
"I don't think McClelland was a top-tier recruit for the Democrats and I don't think the Democrats prioritized this race nationally. The people in that district are probably more economically populist than [Rothfus], but at the same time the district now votes pretty strongly for Republicans at the presidential level."
Even the ultra-conservative Club for Growth, which endorsed Rothfus and spent $317,000 in his successful race against Critz, isn't worried about the race.
"In general, we typically try to avoid supporting incumbents" because they tend to have a fundraising advantage, says Club for Growth spokesman Barney Keller. Still, he says, "I don't think [anyone] rates this race as competitive."