Republican Sen. Lindsey Graham on Thursday warned President Trump not to take any action against the attorney general or the special counsel investigation Russian interference in the 2016 election, saying it could be the “beginning of the end” of his presidency.

“If Jeff Sessions is fired there will be holy hell to pay,” Graham (R-SC) said of reports that Trump is upset with his attorney general for recusing himself from the Russia probes and that he is mulling dumping him.

Graham, a former Republican presidential candidate during the 2016 run, also cautioned the president not to go after former FBI director Robert Mueller, who’s looking into whether Trump campaign officials colluded with the Russians.

“Any effort to go after Mueller could be the beginning of the end of the Trump presidency – unless Mueller did something wrong. Right now I have no reason to believe he’s compromised,” Graham said.