Sen. Lindsay Graham said he is “100 percent behind” embattled Attorney General Jeff Sessions, and said there would be “holy hell to pay” if President Donald Trump fires him.

In an interview with CNN, the South Carolina Republican also said that if the president went after special counsel Robert S. Mueller III, who’s directing the investigation into possible contacts between Trump’s circle and Russia, that could be the “beginning of the end of the Trump presidency, unless Mueller did something wrong.”

Graham said that while he often disagrees with Sessions, he never doubts his integrity. He said Trump’s bullying of the attorney general is not playing well with Republican senators, most of whom have ties to their former Senate colleague.

“This effort to basically marginalize and humiliate the attorney general is not going over well in the Senate,” or the “conservative world,” Graham said.

The roots of Trump’s conflict with both Sessions and Mueller lie in the Russia investigation. Trump first became angry at his attorney general for recusing himself from the probe, due to his own Russia contacts, clearing the way for Trump to be investigated by someone who is not one of his closest allies.

Since then, Trump has regularly excoriated the attorney general on social media, generally over the Russia inquiry.

Since Mueller was appointed to run the investigation, Trump has castigated him over whether the probe would touch on the president’s finances.

While noting that Trump has the right to fire anyone in his Cabinet, Graham said the 45th president should show respect for Sessions “as a human being,” and as one of Trump’s most loyal supporters. Sessions was the first senator to endorse Trump during his presidential run.

Senate Majority Whip John Cornyn expressed similar sentiments Wednesday, saying that if Trump fired Sessions it could be the end of Senate Republicans’ cooperation with the president. It was a rare incident of a congressional GOP leader rebuking Trump. 

On Tuesday, Graham reportedly called Sessions to offer his support and told him to “hang in there.” Graham said the next day that Trump’s attempt to push Sessions to resign rather than firing him outright showed “weakness.”