Should President Donald Trump have special counsel Robert Mueller fired, it could be “the beginning of the end of the Trump Presidency.”

That’s according to Sen. Lindsey Graham (R-S.C.), who on Thursday explicitly warned the president against meddling with the investigation into possible collusion between the Trump campaign and Russia during the 2016 election.

“The idea that the president would fire Mueller ― or have somebody fire Mueller ― because he doesn’t like Mueller or Mueller is doing something he doesn’t like... then we become Russia,” Graham said in an interview on Capitol Hill. “Any effort to go after Mueller could be the beginning of the end of the Trump presidency unless Mueller did something wrong.”

The senator said he and a bipartisan group of lawmakers plan to introduce legislation next week that would prevent Trump, and any future president, from firing a special counsel without judicial review.

“The president’s not in the business of drawing red lines when it comes to the law,” added Graham. “The law is above any presidential red line.”

Trump has previously stated that if Mueller investigated his family’s finances, it would cross a “red line.” Mueller is looking into Trump’s business transactions nonetheless, according to a report last week from Bloomberg, citing an anonymous person familiar with the investigation.

While Trump stopped short of specifically saying he would consider firing Mueller, several media outlets have reported that the president has discussed the topic with White House aides.

Graham also used his statement Thursday to defend Attorney General Jeff Sessions, whom Trump has repeatedly criticized in public as a result of the Justice Department leader’s decision to recuse himself from the Russia investigation.

“If Jeff Sessions is fired there will be holy hell to pay,” he promised.

“This is not just a diversion, this is unnerving,” he said of Trump slandering his own attorney general. “Some of the suggestions the president is making go way beyond what’s acceptable in a rule of law nation.”

“This is not draining the swamp. What he’s [doing] is turning democracy upside down.”