Al Hoffman Jr., a Florida-based real estate developer who was a leading fund-raiser for George W. Bush’s campaigns, said he would seek to marshal support among other Republican political donors for a renewed assault weapons ban.

“For how many years now have we been doing this — having these experiences of terrorism, mass killings — and how many years has it been that nothing’s been done?” Mr. Hoffman said in an interview. “It’s the end of the road for me.”

Mr. Hoffman announced his ultimatum in an email to half a dozen Republican leaders, including Jeb Bush and Gov. Rick Scott of Florida. He wrote in the email that he would not give money to Mr. Scott, who is considering a campaign for the Senate in 2018, or other Florida Republicans he has backed in the past, including Representative Brian Mast, if they did not support new gun legislation.