Sen. Ted Cruz on Thursday gave his supporters an idea of who he might pick as a Supreme Court justice while campaigning for president with Utah Sen. Mike Lee.

"I think Mike would look pretty good in a robe," Cruz said during a campaign stop in South Carolina on Thursday.

It's rare for a sitting senator to move to the high court, though not unprecedented. Franklin Delano Roosevelt was the last president to pick such a nominee. But Lee and Cruz have been very close allies throughout their time in the Senate, evidenced most notably by their attempt to defund Obamacare, which ended in a government shutdown.

And Cruz has made the issue of judicial nominations a key plank in his presidential campaign following the death of Supreme Court Justice Antonin Scalia.

Lee and Cruz both worked as law clerks at the Supreme Court, and Lee has kept his hand in judicial matters since arriving in the Senate. In 2013, he wrote a book arguing that Supreme Court Chief Justice John Roberts' decision to uphold Obamacare's individual mandate as a tax amounted "to a betrayal of his judicial oath."

Cruz has been trying to convince voters that he is better equipped to pick Supreme Court justices than Donald Trump. On Wednesday, he faulted Donald Trump for saying that his sister, U.S. Third Circuit Court of Appeals Judge Maryanne Trump Barry, "would be phenomenal" on the Supreme Court.

"Now, listen, it's great to support your sister, that's a nice thing," Cruz said during a CNN town hall on Wednesday night. "But, if you're actually suggesting someone for the Supreme Court, his sister is a radical pro-abortion judge. She struck down New Jersey's ban on partial-birth abortion as irrational. That's an extreme position. That's who he suggested as a Supreme Court Justice last year."

Trump said his comments about his sister shouldn't be taken seriously. "I said it jokingly," he told ABC's George Stephanopoulos on Sunday. "My sister's a brilliant person, known as a brilliant person, but it's obviously a conflict. And I said, oh, how about my sister? Kiddingly. My sister, also she ... she also happens to have a little bit different views than me, but I said in that in a very joking matter, and it was all lots of fun and everything else. I would say total conflict of interest as far as my sister."

Cruz might shower praise on Lee, but the Utah senator is remaining neutral in the presidential primary; in addition to campaigning with Cruz, Lee hit the trail with Sen. Marco Rubio, R-Fla., this week.

"If either Ted or Marco were the only one of my friends in there, I'd endorse either one," Lee told the Washington Examiner. "If it came down to a two-person race, say, between Cruz and Trump or Rubio and Trump, then it's an easy decision to make."

Cruz is currently second in the Washington Examiner's presidential power rankings, while Rubio is third and Trump is first.