Graham, who dropped his own bid for the White House last December, has criticized Trump consistently since he threw his name in the hat for the nomination in June 2015.
Most notably, Graham criticized Trump for questioning Sen. John McCain's record as a war hero because McCain was captured during the Vietnam War. Trump claimed during a rally last July that Graham called him, "begging" for a reference to get him in on Fox News' "Fox & Friends" program. As proof of his claims, Trump gave out Graham's cellphone number.
Graham has not been alone in rebuking Trump, although no politician has gone further so far to condemn the presumptive nominee.
At a press conference on Tuesday, House Speaker Paul Ryan (R-Wisconsin) admitted that Trump's comments equated to "the textbook definition of a racist comment," but followed up by saying he didn't think a vote for Clinton was a viable alternative.
Trump has repeatedly criticized Curiel, who was born in Indiana, because "he's a Mexican. We're building a wall between here and Mexico."
In an interview with CNN's Jake Tapper, Trump denied that his comments were racist after attempting to dodge the anchor's question 23 times.
In a call with surrogates Monday first reported by Bloomberg News, Trump told supporters to double down on his comments against the judge.
He doubled down on the comments himself Sunday, telling CBS' "Face the Nation" that it was possible a Muslim judge could be biased against him because of his controversial proposal to bar Muslims from entering the United States.
Despite the criticism, it's unlikely that Curiel will be removed from the case.
As NPR noted Saturday, Trump's lawyers have not filed a motion to have Curiel recused, "undoubtedly because court precedents are unanimous in holding that race, ethnicity, gender, religion and sexual orientation are not themselves grounds for disqualifying a judge. If they were, legal ethicists observe, the legal system would fall into chaos because no judge would be free from taint."