"Claiming a person can't do their job because of their race is sort of like the textbook definition of a racist comment," Ryan continued.
Defending his endorsement of the presumptive GOP nominee, Ryan said Trump is more likely to back the House GOP's election-year policy agenda than Hillary Clinton.
"Do I believe Hillary Clinton is the answer? No, I do not," Ryan said. "I believe that we have more common ground on policy issues of today and we have more likelihood of getting our policies enacted with him than we do with her."
Trump has questioned whether Curiel, who was born in Indiana to Mexican immigrants, would be impartial in the Trump University cases because of the real estate mogul's proposal to build a wall along the length of the U.S.-Mexico border.
Trump has also said it's "possible" a Muslim judge wouldn't be impartial because of his call to temporarily bar Muslim immigration to the United States, which he has since called a "suggestion."
Ryan was in Washington's struggling Anacostia neighborhood today to unveil anti-poverty proposals in the House GOP's election-year agenda, taking part in a roundtable and meeting with members of the community.
The 35-page proposal — which featured language that could garner bipartisan support, as well as proposals the White House and Democrats oppose — is the first plank in the six-part agenda, which Ryan is using to refocus 2016 on policy.
But Ryan admitted that Trump's comments undercut Republican efforts.
"I do think these kinds of comments undercut these things. I'm not even going to pretend to defend them. I'm going to defend our ideas," he said.
"I don't know what's in his heart," Ryan said of Trump.
From Ryan to rank-and-file members of Congress, Republicans across the country have found themselves in a difficult position after Trump's comments as they work to come together for the general election while keeping their distance from the New York businessman's most incendiary pronouncements.
"I know Donald Trump. I've known him for 14 years. And Donald Trump is not a racist," Christie said, adding that Ryan is "entitled to his opinion."
In an interview with CNN, Rep. Lee Zeldin, R-N.Y., condemned Trump's comments but did not call him a racist.
"I believe there's more than just words to define a person, and by the way, aside from words, there's a whole lot more to define everyone, but you can easily argue that the president of the United States is a racist with his policies and his rhetoric," Zeldin said, though adding that he was not calling President Barack Obama a racist.
Sen. Lindsey Graham, R-South Carolina, told ABC News Trump is "playing the race card."
"Mr. Trump's attack on the judge is way over the top, it steps on separation of powers, it's unfair, and has a racist element to it," he said.
“I'd love to be able to support my nominee, but I think Mr. Trump takes the country, the party in the wrong direction and I think [Clinton will] be the third term of a failed presidency,” Graham added.