Fed-up leaders are mounting a counterattack against other Republicans sniping at their party’s presidential nominee, Donald Trump, saying the GOP should fire anyone caught bad-mouthing the ticket.

Oregon Republican Chairman Bill Currier will ask his state party’s central committee to formally petition Republican National Committee Chairman Reince Priebus to discipline party staff, consultants or vendors who speak or work against their candidate.

“Trump won the nomination in fair competition, and now that he is the nominee, the RNC should support him, including those we hire at every level,” said Mr. Currier. “If we don’t do that, we have strayed from our mission.”

He said some response is needed after a surge of reports of discontent within the Trump campaign and the broader Republican Party as Mr. Trump’s poll numbers tumble.