Former Republican presidential nominee Mitt Romney took to Twitter on Monday morning to attack President Donald Trump, using the Martin Luther King Jr. holiday to suggest that Trump is a racist.

Romney wrote: “The poverty of an aspiring immigrant’s nation of origin is as irrelevant as their race. The sentiment attributed to POTUS is inconsistent w/ America’s history and antithetical to American values. May our memory of Dr. King buoy our hope for unity, greatness, & ‘charity for all.'”

The poverty of an aspiring immigrant’s nation of origin is as irrelevant as their race. The sentiment attributed to POTUS is inconsistent w/ America’s history and antithetical to American values. May our memory of Dr. King buoy our hope for unity, greatness, & “charity for all.” — Mitt Romney (@MittRomney) January 15, 2018

The former Massachusetts governor was referring to allegations that President Trump had referred to “shithole countries” in a negotiation over immigration policy in the Oval Office last Thursday with a small group of Senators that included Sen. Dick Durbin (D-IL).

Trump has denied using that term, and Sens. Tom Cotton (R-AR) and Frank Purdue (R-GA) have denied hearing it, as has Department of Homeland Security Secretary Kirstjen Nielsen, who was also present.

The White House said that President Trump was arguing for a merit-based system of immigration policy, as opposed to the existing policies of “chain migration” and the “diversity visa” lottery.

The president was also reportedly defending his administration’s decision to send some El Salvadorans and Haitians, who are in the U.S. due to earthquake disasters, back to their countries of origin after their temporary protected status expires.

Democrats have also used the Martin Luther King Jr. holiday to attack Trump, based on the recent controversy and on past claims that he is a racist.

Speculation has mounted whether Romney would be a friend or foe of President Trump if elected to the Senate.

Romney has a rocky relationship with President Trump. In March 2016, he viciously attacked Trump in a speech in Salt Lake City, Utah — but stopped short of endorsing a rival candidate in the Republican primary. He mended fences with Trump when he aspired to become Secretary of State in the new administration, but has since re-emerged as one of the president’s leading GOP critics, appealing to the remnant of the “NeverTrump” movement.

Last summer, Romney criticized President Trump’s response to violence in Charlottesville, accusing him of moral equivalence between neo-Nazis and those opposing “racism and bigotry” — but without noting that the latter had disrupted a lawful, permitted protest.

Democrats accused Romney himself of racism when he ran for president in 2012, particularly when he accused Barack Obama supporters of voting for “free stuff,” and when he joked that “no one’s ever asked to see my birth certificate.”

Joel B. Pollak is Senior Editor-at-Large at Breitbart News. He was named to Forward’s 50 “most influential” Jews in 2017. He is the co-author of How Trump Won: The Inside Story of a Revolution, is available from Regnery. Follow him on Twitter at @joelpollak.