More than two-fifths of Americans believe civil war is at least somewhat likely in the next 10 years, according to a new survey – a figure that increases to more than half among self-identified “strong Republicans”.

Amid heated rhetoric from supporters of Donald Trump, the findings, in research by YouGov and the Economist, follow similar results in other polls.

On Sunday night, the South Carolina senator Lindsey Graham predicted “riots in the streets” if Trump is indicted over his retention of classified documents after leaving the White House, materials recovered by the FBI at Trump’s home this month.

Graham earned widespread rebuke. On Monday, Mary McCord, a former acting deputy attorney general, told CNN it was “incredibly irresponsible for an elected official to basically make veiled threats of violence, just if law enforcement and the Department of Justice … does their job”.

Saying “people are angry, they may be violent”, McCord said, showed that “what [Trump] knows and what Lindsey Graham also knows … is that people listen to that and people actually mobilise and do things.