Donald Trump warned evangelical leaders that if Republicans lose control of Congress in the midterm elections, Democrats will institute change "quickly and violently," The New York Times has reported.

At a meeting with those leaders at the White House on Monday, the US president said everything was at stake for his conservative agenda if his party loses in November, according to an audiotape of the meeting obtained by the Times.

Democrats "will overturn everything that we've done and they'll do it quickly and violently," Trump said, according to the Times report published Tuesday night.

"They will end everything immediately."

"When you look at Antifa," he added, referring to militant Leftist anti-fascism groups, "and you look at some of these groups, these are violent people."