Conor Lamb, center, the Democratic candidate for the March 13 special election in Pennsylvania's 18th Congressional District, and former Vice President Joe Biden, right, pose for a selfie with a supporter during a rally at the Carpenter's Training Center in Collier, Pa., Tuesday, March 6, 2018. (AP Photo/Gene J. Puskar)

Conor Lamb, center, the Democratic candidate for the March 13 special election in Pennsylvania's 18th Congressional District, and former Vice President Joe Biden, right, pose for a selfie with a supporter during a rally at the Carpenter's Training Center in Collier, Pa., Tuesday, March 6, 2018. (AP Photo/Gene J. Puskar)

PITTSBURGH (AP) — Embracing the Democratic Party’s next special election challenge, former Vice President Joe Biden on Tuesday called on western Pennsylvania’s labor unions to rally behind Democrat Conor Lamb in a contest expected test the loyalty of President Donald Trump’s working-class coalition.

Biden, who at 75 remains one of the Democratic Party’s biggest stars, declared that the little-known Lamb would “throw himself in front of a train” to protect the working class from Republican-backed plans to cut Social Security and Medicaid. The 33-year-old Lamb, Biden said, also reminded him of his late son, Beau.

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“We need him to bring this country back together again,” the former vice president said of Lamb as he faced union carpenters in a packed suburban Pittsburgh union hall. He added, “Conor knows what the people of this district need.”

The March 13 special election to replace the scandal-stained Republican Rep. Tim Murphy will likely be viewed as a referendum on Trump and his Republican Party, who are eager to avoid another embarrassing special election loss in a contest the GOP should win — on paper at least.

The president won Pennsylvania’s 18th Congressional District by nearly 20 percentage points little more than a year ago. Yet polls suggest the race is essentially tied one week before Election Day.

Reflecting the high political stakes, outside groups on both sides have flooded local airwaves with advertising. And each party has called in its superstars to energize voters.

Trump was expected to visit the area Saturday on behalf of the Republican candidate, state Rep. Rick Saccone.

There have been several elections since Trump’s 2016 victory, but none has tested the loyalty of the white working-class voters who fueled his winning coalition across the industrial Midwest more than this one.

The western Pennsylvania district, which stretches from suburban Pittsburgh to the West Virginia border, is overwhelmingly white and features an estimated 17,000 steelworkers.

“We started as a divided fragmented group,” Darrin Kelly, president of the Allegheny County Labor Council, said before Biden and Lamb took the stage. “Today, as we come to the finish line, we are a united unstoppable army.”

A native son of working-class Scranton, Pennsylvania, Biden worked to ensure that the union “army” would unite behind Lamb and the Democratic Party once again.

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“He’s not afraid to say the word ‘union,’” Biden said of Lamb at a subsequent appearance at a suburban Pittsburgh university. He told the carpenters: “It makes me angry when we’re not respected — when you’re not respected.”

Lamb, too, offered warm words for his would-be labor allies in next week’s contest.

“You are the heart and soul of this campaign,” he said.