PITTSBURGH — Stepping into Pennsylvania’s high-profile special election, former Vice President Joe Biden on Tuesday called on labor unions to support Democrat Conor Lamb in next week’s contest that he said pits the values of the “super wealthy” against the working-class.

Biden, who even 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 working-class voters from GOP plans to cut programs like Social Security and Medicaid.

The former vice president delivered his message inside a Western Pennsylvania union hall as he worked to rally union members just seven days before the nation’s next Trump-era special election.

While Biden did not mention President Donald Trump by name, the March 13 contest will likely be viewed as a referendum on the president and his Republican Party.

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

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

Biden said Lamb’s opponents are spending millions of dollars against him to protect the recently passed tax overhaul that largely benefit “the super wealthy.”

Biden’s union hall appearance reflected the significance of white working-class voters in next week’s special election. The district, which stretches from suburban Pittsburgh to the West Virginia border, is overwhelmingly white and features an estimated 17,000 steelworkers.

Polls suggest the race is a tossup.