U.S. Sens. Sherrod Brown and Bob Casey, two of the country’s leading Democratic allies of organized labor, stumped for Democrat John Fetterman on Thursday in Monaca, saying voters should elect him to join their caucus as a key line of defense against a GOP that — they say — wants to strip away workers’ rights.

At a gathering of union members at a small Monaca event hall, labor leaders encouraged voters to elect candidates who value a worker’s right to unionize, and the Democrats responded — in no uncertain terms — that their party is the one to do it.

Making his first stop on a swing through Pennsylvania, Mr. Brown, D-Ohio, said when Mr. Fetterman and Democrat Tim Ryan in his state are elected to the Senate, it’ll “move this country in a more progressive, pro-labor direction.” Mr. Fetterman, the sitting lieutenant governor, is facing Republican Mehmet Oz, a cardiothoracic surgeon and TV celebrity, in the November contest.

“We want to make sure that we fight for Fetterman, who will stand up to Wall Street, stand up to corporate America and stand up to the oil companies and drug companies,” Mr. Brown said. Mr. Fetterman has repeatedly criticized companies like Shell and Chevron for raking in record-breaking profits while consumers have been paying higher prices at the pump.

Mr. Fetterman, in a short speech, talked up the “union way of life” and noted that he lives across the street from a steelworkers local. He said if elected, he will work to eliminate the filibuster, raise the minimum wage — he favors $15 an hour — and pass the PRO Act.