When Stacy Garrity rallied supporters in McCandless in late September, she acknowledged it “won’t be easy” to oust Gov. Josh Shapiro. The Republican state treasurer’s uphill battle next year against a widely popular governor got even steeper on Tuesday, when Democrats racked up wins all over the ballot and across Pennsylvania and the U.S. — including in races where Mr. Shapiro campaigned.

Democratic tussling over the best paths to victory against a Republican Party dominated by President Donald Trump didn’t matter much, political experts told the Post-Gazette. Moderates and left-wing candidates alike won decisively — from swing counties to longtime Trump strongholds across the state and country.

While Republicans point out that it’s still early and Ms. Garrity is in a strong position to build name recognition and win over voters, Democratic strategists, pollsters and academics say the dominant performance bodes well for Mr. Shapiro and puts the spotlight on a tough environment not just for Ms. Garrity but for the GOP at large.

“What happened on Tuesday night wasn’t just an old fashioned butt-kicking,” said J.J. Balaban, a Democratic strategist and admaker based in Philadelphia. “It was an old fashioned butt-kicking in every part of the state.”

Mr. Shapiro and several Democrats cited wins in a range of offices in counties previously won by Trump. These include Erie, Washington, Westmoreland, Beaver, Bucks, Luzerne, Northampton, Cumberland and more. “If there was any good news for Republicans in Pennsylvania on Tuesday, I haven't found it yet,” Mr. Balaban said.