At a gathering in a tavern outside Philadelphia on Monday evening, supporters of Scott Wallace, a Democrat running in the state’s most hotly contested House race, denounced Mr. Trump for his “cruelty” and alluded repeatedly to the president’s rhetoric on race and national identity. Addressing a tightly packed crowd, former Representative Patrick Murphy, a Democrat who used to represent the area, warned that “people who hate feel so emboldened to act on it.”

The suburbs around Philadelphia used to be a reliable Republican bastion. But Shelley Howland, a Republican who attended the pro-Wallace event, said Mr. Trump represented a breaking point.

A supporter of abortion rights and gun control, Ms. Howland voted two years ago for Hillary Clinton over Mr. Trump, but stayed loyally Republican in the congressional election, supporting Mr. Wallace’s opponent, Representative Brian Fitzpatrick, who is now seeking his second term. She said she would not support Mr. Fitzpatrick again.

“This year, it’s going to be a straight Democratic ticket,” said Ms. Howland, 65, lamenting “this whole movement to the alt-right, Steve Bannon in the White House, Trump in the White House.”