GREENSBURG — In a sweep through the blue-collar counties of southwestern Pennsylvania on Tuesday that ended on the patio of a brewery, Lt. Gov. John Fetterman said the Democratic Party cannot afford to turn its back on any of the state’s 67 counties — no matter how red they are — if it wants to flip one of the country’s most important U.S. Senate seats this November.

Mr. Fetterman, the front-runner who is running against U.S. Rep. Conor Lamb and state Rep. Malcolm Kenyatta in next Tuesday’s Democratic primary, said it’s not about turning the reddest counties blue, but about trimming into the margins here and halting the enthusiasm behind the eventual GOP nominee.

Democrats in red counties are the soul of the party, Mr. Fetterman said, and the winner of next week’s primary will need every county and every vote.

“Is there a new law that says a vote in Greensburg counts less than a vote in Pittsburgh?” Mr. Fetterman asked the crowd at El Diablo Brewing Co. & Wood Fired Kitchen. “Your votes count the same everywhere. That’s why we go everywhere.”