Sen. Susan Collins became the latest Republican lawmaker on Monday to condemn Donald Trump's ongoing attacks against a federal judge of Mexican heritage.

"His statement that Judge [Gonzalo] Curiel could not rule fairly because of his Mexican heritage does not represent our American values," the Maine senator said in a statement.

She continued, claiming that Trump's criticism of the U.S. District Court judge "demonstrates both a lack of respect for the judicial system and the principle of separation of powers."

Hours before Collin's condemnation, Trump refused to apologize for claiming Curiel's "Mexican heritage" is an "inherent conflict of interest" and he should thus recuse himself from handling a civil fraud case that involves Trump's now-defunct online university.

Over the weekend, the presumptive Republican nominee said he would feel similarly if a Muslim judge was presiding over the case.

Trump's criticism of Curiel has drawn the ire of a handful of top Republicans who view it as yet another setback in the billionaire's attempt to make inroads with Latino voters. Like Collins, other GOP critics have accused Trump of waging an attack on American ideals.

"This is one of the worst mistakes Trump has made," former House Speaker Newt Gingrich said on Fox News Sunday, adding that Curiel "is an American. Period."

'When you come to America, you get to become an American, and Trump, who has grandparents who came to the U.S., should understand this as much as anybody," he added.

House Speaker Paul Ryan, who endorsed Trump last week, has said he "completely disagree[s]" with the billionaire's "thinking" on the matter and Senate Majority Leader Mitch McConnell claimed such attacks "don't serve [Trump] very well at this point."

Collins, who supported former Florida Gov. Jeb Bush in the primary, has previously vowed to support the GOP nominee, but has not publicly endorsed Trump since he secured the necessary delegates to clinch the nomination.

She told a radio program last month that Trump "must stop with the gratuitous personal insults" if his sincere goal is to unite Republicans ahead of the November elections.