John Hancock, a Republican strategist in Missouri, worries that emphasizing the Confederacy and racial strife could have long-term negative consequences for the party.

“Demographics aren’t political, they just are facts, and there aren’t enough people that look like me to elect candidates to office, and that characteristic is increasing in the country,” said Mr. Hancock, who is white. “So you have got to have a broader cross-section of support if you want to win elections—you’d better appeal to a rapidly growing Hispanic population in the U.S. and the African-American population.”

Scott Reed, senior political strategist at the U.S. Chamber of Commerce, said: “The White House needs to stop the unpresidential distractions like this week’s tweets,” referring to Mr. Trump’s posts about Nascar and the Confederate flag, among other issues…

“It’s dark days right now, and anybody who says it’s not dark days is not being honest,” said a GOP Senate campaign aide. “But he can turn it around. He just needs to stop [expletive] tweeting about Confederate flags.”