President Donald Trump called Georgia’s Republican Gov. Brian Kemp a “moron” and a “nut job” in a recent disturbing phone call, sources have told CNN.

The increasingly ugly attacks on Kemp by Trump are heightening fears that the president could threaten the chances of Georgia Republican Senate candidates in the upcoming runoff election. Nerves are especially frayed with Trump’s planned rally Saturday in Georgia amid concerns about what might come out of his mouth.