U.S. President Obama underscored that he stands firmly alongside Muslims in America against those who "seek to divide" the country — an apparent reference to presumptive Republican presidential nominee Donald Trump.

Speaking on the eve of the start of the Muslim holy month of Ramadan, Obama also vowed to continue welcoming mostly Muslim immigrants and refugees, Politico reports.

"I stand firmly with Muslim American communities in rejection of the voices that seek to divide us or limit our religious freedoms or civil rights," Obama said. "I stand committed to safeguarding the civil rights of all Americans no matter their religion or appearance. I stand in celebration of our common humanity and dedication to peace and justice for all."

Trump has pledged to not only crack down on border security but also impose a temporary ban on Muslims entering the country.

Obama announced that he will be hosting Muslims at the White House during Ramadan, which begins on Monday, June 7.

The president made his remarks after Trump recently suggested that a judge of Mexican heritage may be biased against him. The judge, Gonzalo Curiel, is hearing a case against Trump concerning complaints against his former Trump University.

Trump reiterated in interviews broadcast on Sunday that Curiel's Mexican heritage means he cannot ensure a fair trial involving a presidential candidate who wants to build a border wall to keep people from illegally entering the United States from Mexico, according to the Associated Press.

Curiel was born in Indiana to Mexican-born parents. This, according to the brash billionaire, makes him "a hater of Donald Trump."

Republican leaders rejected Trump's veiled attack on Curiel, with one of them urging him to start acting like "a potential leader of the United States."

"We're all behind him now," Senate Majority Leader Mitch McConnell said, adding that it's time for unifying the party, not "settling scores and grudges."

"I don't condone the comments," added Sen. Bob Corker, chairman of the Senate Foreign Relations Committee, on ABC's "This Week."

Newt Gingrich, who is reportedly on the shortlist of Trump's possible running mate, delivered the harshest warning of all.

"This is one of the worst mistakes Trump has made. I think it's inexcusable," Gingrich, a former presidential contender, said on "Fox News Sunday."

Trump University is the subject of two lawsuits — one in San Diego and the other New York — both accusing the school of fleecing students with unfulfilled promises to teach secrets of success in real estate.

Trump denied the accusation, saying the graduates of his university were overwhelmingly satisfied with the education they got.