Pressure from high-profile Republicans and rank-and-file voters is mounting to reject Donald Trump's candidacy amid fallout from him saying gun rights activists could stop Hillary Clinton nominating liberal US Supreme Court justices.
Nearly one-fifth of 396 registered Republicans in a Reuters/Ipsos August 5-8 poll want Mr Trump to drop out of the race for the White House and another 10 percent "don't know" whether the Republican nominee should or not.
MSNBC host Joe Scarborough, a former Republican congressman from Florida, wrote an opinion piece in the Washington Post saying the party was in "uncharted waters" and called for leaders to start looking for ways to remove Trump from the ticket.
John Negroponte, former director of national intelligence under President George W Bush, and former Republican US Representative Chris Shays were among those that announced their support on Wednesday (local time).
"Donald Trump lost me a long time ago," Shays told MSNBC in an interview.
"He does and says everything my mom and dad taught me never to say and do. He doesn't understand the basic requirements of being president of the United States. And, frankly, he's dangerous."
Ms Clinton's campaign now has a website for Republicans and political independents to sign up in support of Ms Clinton.
Mr Trump, a New York businessman, was seeking to reset his campaign this week with an economic policy speech after a series of missteps that included a prolonged clash with the parents of fallen Muslim American Army Captain Humayun Khan.
But Mr Trump's remark at a Tuesday rally that gun rights activists could prevent Clinton from placing liberal justices on the US Supreme Court immediately sparked a torrent of criticism on social media that he was effectively calling for Clinton's assassination.
"If she gets to pick her judges, nothing you can do folks," Mr Trump said at the rally at the University of North Carolina.
"Although the Second Amendment people, maybe there is, I don't know," he continued.
The US Constitution's Second Amendment guarantees a right to keep and bear arms.
Ms Clinton's campaign called Mr Trump's remark "dangerous".
Mr Trump's campaign said the comment was misinterpreted and that he was encouraging gun activists to use their political power.
"What he meant by that was you have the power to vote against her," former New York Mayor Giuliani said.
Mr Trump's comment and the resulting backlash occurred as Reuters/Ipsos polling showed already deep divisions among US voters over Mr Trump's candidacy.
In addition to the poll showing 19 percent of registered Republicans want Mr Trump to drop out, a separate Reuters/Ipsos tracking poll showed that some 44 percent of 1162 registered voters believe he should exit the race.