Cruz has turned on his old friend in recent months. REUTERS/Brian Snyder

ST. LOUIS -- As a young lawyer, Sen. Ted Cruz (R-Tex.) used to emulate the argument style of now-Supreme Court Chief Justice John Roberts and invited him to be part of the legal team working on the Florida recount for George W. Bush.

Fifteen years later, Cruz no longer wants to imitate Roberts. Ever since Roberts wrote an opinion upholding the Affordable Care Act, Cruz has been slamming the chief justice, and mused Saturday on imagining a Supreme Court without him.

Cruz, a former Supreme Court clerk who tailored his law school experience to get the job, has made criticizing the high court a central part of his presidential campaign. The Texas Republican, who is trying to gain the support of conservatives and evangelical Christians, soured on the court after the ACA decision and after it ruled in June that same-sex marriage is legal nationwide.

[Read: Cruz once clerked for a chief justice, but he’s no longer a friend of the court]

In the past few months, Cruz has taken particular aim at Roberts, whom Cruz wrote in his book "A Time for Truth" was "the best Supreme Court advocate of your generation." Roberts wrote the opinion upholding the health-care law; Cruz wants to repeal the law. Saturday, while addressing the conservative Eagle Forum's conference in St. Louis, Cruz asked the room to imagine what America would be like if Roberts and retired justice David Souter were never on the court. In the past, Cruz has also criticized Justice Anthony Kennedy.

"In 1990, in one room, was sitting David Souter," said Cruz. "In another room was sitting Edith Jones, the rock-ribbed conservative jurist of the Fifth Circuit Court of Appeals. George Herbert Walker Bush picked David Souter. Let's fast-forward to 2005. In 2005, in one room was John Roberts, and in another room was my former boss, Mike Luttig, a rock-ribbed conservative jurist on the Fourth Circuit Court of Appeals -- and George W. Bush picked John Roberts."

A man in the crowd muttered his disapproval: "Thank you, Bushes."

Cruz explained that he was not questioning the Bushes' conservatism. "It's that it was easy," he said. "Neither Souter nor Roberts had said much of anything. They didn't have a paper trail. They wouldn't have much of a fight. Whereas if they actually nominated a conservative, then they would have had fights. Let me tell you the difference of that. If instead of David Souter and John Roberts, the Presidents Bush had nominated Edith Jones and Mike Luttig, Obamacare would have been struck down three years ago, and the marriage laws of every state would still be on the books."

Roberts dissented in the same-sex marriage decision; Souter retired in 2009, before either the ACA or gay marriage ruling.

In 2005, Cruz wrote a piece in National Review praising Roberts as a "mainstream judge" and said that as a jurist, "Judge Roberts’s approach will be that of his entire career: carefully, faithfully applying the Constitution and legal precedent."

In an interview in July with The Washington Post, Cruz, who has called for Supreme Court justices to be subject to judicial retention elections, said he and Roberts have been friends for 20 years.

[Read: Ted Cruz calls for judicial retention elections for Supreme Court justices]

“His decisions on the two Obamacare cases have been heartbreaking . . . because he’s a talented enough lawyer that he knows exactly what he’s doing,” Cruz said.

Cruz has also been using his involvement with the Supreme Court to his advantage. On the campaign trail, he touts cases he was involved with that won by a 5-4 vote on the court. One involved the Ten Commandments at the Texas state capitol and another a cross at a Mojave Desert veterans memorial.

“You want to know what this election is about?" Cruz asked a "Rally for Religious Liberty" he held in Des Moines last month. "We’re one justice away from the Supreme Court saying every image of God shall be torn down.”