"If he stays within the four corners of the document, what good is his testimony? If he goes outside the document, he's going to get killed politically, because then he becomes a political commentator. If he does what he said he would do in his press conference, refer back to the document, this is just a political circus," Graham said Wednesday.
Mueller agreed to testify before the House Judiciary and Intelligence Committees on July 17 after they subpoenaed the special counsel on Tuesday, according to the committees' chairmen, Reps. Jerry Nadler, D-N.Y., and Adam Schiff, D-Calif.
Graham said Dems' push for Mueller to testify is not about the "truth," but rather about "impeachment."
The senator said the latest developments hurt House Speaker Nancy Pelosi, D-Calif., the most.
"The biggest loser of Mueller coming into the House, I think, is Nancy Pelosi. It is going to be harder for her to control her troops after Mueller testifies," Graham said.
Graham also commented on a question asked by President Trump regarding the Mueller testimony in a call to Fox Business News.
"The president asked a really good question. 'When does this end?' Mr. President, this ends with your re-election. Once you win in 2020, this ends. But between now and then, they are going to do everything they can to undermine your presidency. They are going to retry the Mueller case," Graham said.