Republican Rick Saccone, left, and Democrat Connor Lamb on Feb. 19 before the taping in Pittsburgh of their first debate in the special election in Pennsylvania’s 18th Congressional District. (Keith Srakocic/AP)

PITTSBURGH — Former vice president Joe Biden said Tuesday that Conor Lamb can accomplish what Democrats have struggled to do in recent elections: win over working-class voters.

Biden campaigned for the Democratic nominee in a tight congressional race in southwest Pennsylvania amid fresh signs of Republican nervousness about holding a district that President Trump won by double digits in the 2016 presidential election.

“If [Lamb] wins you’re probably going to see another half-dozen Republicans say they’re not running again,” Biden said as he and the candidate shook hands at a union apprenticeship shop in the Pittsburgh suburbs. “He has the capacity to do what I’ve spent my whole life doing, and that is getting back working-class people supporting us again, because he cares about them.”

Within hours of Biden’s appearance, the National Republican Campaign Committee plowed another $619,664 into the race, bringing its total ad buy to more than $3.5 million. The latest ad by the conservative group sought to portray Lamb as weak on crime by criticizing plea deals that some drug dealers and gun dealers got when Lamb worked as a federal prosecutor.

The Trump administration has been stepping up its efforts to rescue the once-safe Republican seat. Trump and top officials will campaign for GOP candidate Rick Saccone within days of Biden’s appearance.

“[Lamb] reminds me of my son Beau,” Biden said at an afternoon rally, referring to his late son, the former Delaware attorney general who died of cancer in 2015. “I hope the Republicans find some candidates — even if it means we lose some races — who are cut from the same cloth.”

On Saturday, Trump will hold a rally for Saccone, a state representative who’s run as an ally of the president.

“When the media asks me, ‘Isn’t President Trump’s popularity waning?’ I tell them, ‘Listen, you’re in Trump country,’ ” Saccone said Monday night at a rally with 60 supporters in a rural part of the district. “When I start to feel sorry for myself, I think of my president and what he’s doing. Everybody seems to be against him, and he needs a lot of help.”

The White House has tried to help Saccone. Trump delivered a jobs speech in the district in January, Vice President Pence attended a February rally and fundraiser, and Secretary of the Interior Ryan Zinke went to a small town in the district to announce $55.7 million for Pennsylvania reclamation projects. Kellyanne Conway, a top White House aide, is holding a rally in the district on Thursday, according to senior administration officials, and Donald Trump Jr. is traveling there Monday for another event.

President Trump’s abrupt announcement last week of proposed tariffs on steel and aluminum looms large in a district with thousands of steel jobs. Both candidates have expressed support for the decision.

The president’s planned trip on Saturday has triggered a debate in the West Wing, with some aides concerned Trump could be linked to a potential loss. But the president, who has made several economic announcements in the state, wanted to go to promote steelworkers. One senior administration official said Trump might even sign the tariffs at the event to generate additional media coverage.

White House officials said that an upside of the tariffs — opposed by many Washington Republicans — is they could potentially help in the congressional district. The 18th District, which will be reshaped this year after Republican-drawn maps were ruled unconstitutional, stretches from Pittsburgh’s wealthier suburbs to the state’s borders with Ohio and West Virginia.

The United Steelworkers union, which has endorsed Lamb, claims nearly 18,000 members in the district, and Pittsburgh-based U.S. Steel was among the companies invited to the White House last week to heap praise on the tariffs.

“Having been somebody that has global views and believes in free trade, we know when it’s completely unfair,” said U.S. Steel CEO David Burritt. “We are not protectionists. We want a level playing field. It’s for our employees, to support our customers.”

At a weekend debate, Lamb said he backed the tariffs on the advice of the United Steelworkers. Like the union, he preferred that tariffs target “the bad actors” while U.S. businesses are protected.

“I do agree that we need to take some action to level the playing field here,” Lamb said. “Our steelworkers have worked hard for a long time, and they make the best steel in the world. For too long, China has been making cheap steel, and they’ve been flooding the market with it. It’s not fair, and it’s not right. So I think it’s long overdue.”

Saccone, who immediately came out in support of the tariffs, emphasized that he trusted the president to get the policy right.

“There’s an imbalance, and President Trump’s trying to level it back out again,” Saccone said. “It’s the first volley in what he’s doing. He’s a negotiator. He’s a master negotiator. You put out the line and maybe there’ll be a little bit of give and take on it, but at the end, you come out with a good deal for America.”

Other issues and tactics have not halted Lamb’s advance. The Democrat, a first-time candidate from a well-known political family, outraised Saccone by a 4-to-1 margin, confirming some early White House angst about the Republican’s fundraising ability. While Lamb said in January that he would not back House Minority Leader Nancy Pelosi (D-Calif.), the GOP’s Congressional Leadership Fund ran millions of dollars in ads trying to tie him to Pelosi.