When Lydia Balogh, a registered Republican, showed up at a rally for Democrat Conor Lamb here in suburban Pittsburgh, she was already in his camp. And she was already doing her part to help his congressional bid, hosting campaign workers in her home.

But she wanted to show her support in person – and see former Vice President Joe Biden, who joined Mr. Lamb on stage in a rousing endorsement of the telegenic young former federal prosecutor and former Marine.

Lamb’s race against Republican state Rep. Rick Saccone in Tuesday’s special election for Pennsylvania’s 18th congressional district is a nail-biter, according to polls – a shocking turn, given that President Trump won the district in 2016 by almost 20 percentage points. This is Trump country, dominated by white, working-class voters, along with wealthier, close-in suburbs. Many issues burn bright, starting with jobs, steel tariffs, workers’ rights, and guns.

But for Ms. Balogh, this race is not primarily about issues.

“It’s about character and integrity, things we desperately need,” says Balogh, who is retired from her job in sales. “It’s time for bipartisanship. We need to work together.”

Lamb and Mr. Saccone are battling to fill the seat vacated by former Rep. Tim Murphy (R), who resigned last October amid reports that he had asked his mistress to have an abortion despite his public stance opposing abortion.

Supporters of Saccone also cite values – often expressed in one word, “freedom” – in explaining their choice. Gun rights loom large, amid renewed efforts to limit access to weapons. Saccone supporters also cite Trump’s tax cuts and efforts to repeal Obamacare as reasons to send the Republican state lawmaker to Washington. After all, Saccone says, he was “Trump before Trump was Trump.”

Keith Srakocic/AP Republican Rick Saccone acknowledges the crowd during a campaign rally with President Trump, Mar. 10, in Moon Township, Pa. Mr. Saccone is running against Democrat Conor Lamb in a special election on March 13 for Pennsylvania's 18th congressional district, a seat vacated by Republican Tim Murphy.

But Saccone isn’t really another Trump. He has run a campaign seen as lackluster, compared with Lamb’s energetic effort, forcing national Republican committees and outside groups to kick in more than $9 million. Trump has come to the district twice, as have other high-profile surrogates. Saccone is a generation older than Lamb, and an experienced politician, while Lamb is a first-time candidate.

The last time Pennsylvania’s 18th district sent a Democrat to Washington was 2003. If Lamb wins, watch gleeful Democrats announce that they have “cracked the code” for how to win in Trump country. A Lamb victory would give rocket fuel to Democrats’ push to retake the House in the November midterms, spurring donations and activism.

Lamb “could give his party a blueprint for how to win swing districts across the country,” says a Republican activist whose political action committee has donated to Saccone.

Even if Saccone wins narrowly, Democrats can claim a victory of sorts, as they have in other unexpectedly close Republican victories in special House races since Trump became president.

The wild card in this southwestern Pennsylvania district is the union vote. Between 20 and 25 percent of the electorate here lives in union households, and “it will probably be greater than that in the special, because organized labor is doing a lot to turn out members,” says Democratic political consultant Mike Mikus, who lives in the district. “So it could hit 30 percent.”

Union voters have trended Republican in recent years, for both cultural and economic reasons. But Democrats believe that a stronger economic message that addresses kitchen-table issues could make a difference.

Regis Ryan, a steamfitter who came to see Lamb and Mr. Biden at a union-sponsored event near Pittsburgh last week, says Trump has made promises to workers that he can’t keep.

“He came to the area and said he’d get their jobs back, but that’s a lie,” says Mr. Ryan, citing the rise of robotics and cheap natural gas as factors behind the changing job picture in this part of Pennsylvania. “There’s a trust factor here; so many people are seeing that.”

And what about gun rights? It’s an issue important to many union members here.

“I’m pro-Second Amendment, but I vote my wallet,” says Ryan, pulling a thick billfold out of his back pocket and slapping it on his palm. He’s volunteering for Lamb.

On most hot-button issues, Lamb and Saccone are actually on the same page. Both support Trump’s steel and aluminum tariffs and gun rights, and both call themselves pro-life, though Lamb says he is willing to leave the law as is.

But it’s on labor issues where the two men part company. Saccone supports “right to work” laws, which block unions from requiring dues payments from workers they represent in collective bargaining. Saccone also supports so-called “paycheck protection” legislation, which makes it hard for public sector unions to raise funds for political activity.

Former Congressman Murphy was a reliably conservative vote in Congress – except on matters affecting unions. Saccone’s departure on labor matters has given Lamb an opening.

‘A prop in the commercial’

Saccone may well look at his Democratic opponent and wonder how a political novice still just starting out in life could grab a reliably Republican seat in Congress. The affable state representative runs through a 40-year career that includes service as an officer in the Air Force, a year as a diplomat in North Korea, two published books, college professor, international businessman, and four terms in the state legislature.

“How do you match that with a young guy like this who has no life experience – a few years in the military working in the legal office, an Obama appointee to the US Attorney’s office for a few years,” Saccone says in an interview at his field office in Greensburg. “That’s it.”

And that ad showing Lamb firing a rifle? “He didn’t have a shooting job” in the military, Saccone says. “It was a prop in the commercial.”

Saccone’s South Korean-immigrant wife, Yong – a flamboyant contrast to her husband with dyed magenta hair and a stars-and-stripes outfit – nudges him to talk about his Italian immigrant roots. The contrast with Lamb, an Ivy League educated scion of a prominent political family, is stark (Lamb’s grandfather was Democratic majority leader in the Pennsylvania state Senate, and his uncle is Pittsburgh’s controller). Saccone suggests that Lamb is trying to pull one over on the people of the 18th district.

“Lamb is a far-left liberal in disguise, trying to pretend that he’s a moderate Democrat,” he says.

Lamb says he won’t vote for Democratic House leader Nancy Pelosi for speaker, if the party retakes the House. But, Saccone says, Lamb has “all her lieutenants around him,” giving him money and coming into the district to campaign, including “crazy Uncle Joe Biden.”

The Lamb campaign declined to make the candidate available for an interview.

A quick walk around Greensburg, whose ornate courthouse dominates a county seat that has seen better days, reveals more bars than coffee houses. In interviews, residents say they’re either not planning to vote, or still undecided.

“I’m probably leaning toward Saccone,” says Kelli, a Trump supporter who tends bar and is about to start working in a bank. “He’s a real person.”

Mike, a self-employed electrical installer, also supports Trump and cares deeply about Second Amendment rights. “Anybody tries to take my guns away – that’s not happening,” he says.

Biden vs. Trump

Recent visits by both Biden and Trump intensified the spotlight on a race that had already captured national attention. Biden spoke emotionally of how Lamb reminded him of his late son Beau – both were military lawyers – and he positioned himself as a son of Scranton, Pa., with blue-collar values that match the 18th district.

Biden sounded, for all the world, like a presidential candidate. Trump, even more so, treated his visit here Saturday night like a presidential campaign event.

Democrats in the 18th district have little to lose. If Lamb falls short on Tuesday, he is already poised to run again in November for a seat in Pennsylvania’s new congressional map. (The current map was rejected by the state’s high court over partisan gerrymandering.)

But for now, the Lamb campaign is keeping its eye on the prize. Hanging on the wall in its field office in Carnegie is a hand-written sign that declares, “Character matters!” Indeed, the workers there were unfailingly polite to two reporters who showed up unannounced.

No matter the outcome, it’s been “a beautiful campaign to watch,” says Joe DiSarro, chairman of the political science department at Washington & Jefferson College in Washington, Pa., and a member of the Republican State Committee.

This district was drawn to favor a Republican, he notes, and even though Lamb is an articulate, attractive candidate, Mr. DiSarro isn’t sure he can pull it off.

“Both men have good qualities,” he says. “Will the voters go with experience, a more mature person who has been in office and has a record? Or do they go for someone who’s a fresh face, has new ideas, and has appealed to voters’ desire to get things done?”

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The 18th district is still very socially conservative, DiSarro notes, and Lamb is running against his party. Saccone is running with his party, and with Trump, who remains reasonably popular here. That calculus probably favors Saccone, despite his anti-union positions. On Sunday, the Pittsburgh Post-Gazette endorsed Saccone.

But if the labor turnout effort is effective, Lamb could win. And that, to quote Biden, would be a big deal.