The incident became a flash point in Mr. Biden’s struggle to address the record number of migrants crossing the southwest border illegally over the last year. Many said the administration’s response was rooted in racial bias, an allegation Alejandro N. Mayorkas, the homeland security secretary, has rejected.The administration did allow more than 8,900 Haitians into the country between September and December to appear in immigration court and request asylum. But during the same period, it also used the public health rule to expel roughly the same number, according to government data, sending them back to a country that the United States considered unsafe for Americans because of kidnapping, crime and civil unrest.“We are concerned that the administration’s use of the Title 42 authority is depriving legitimate asylum seekers the opportunity to pursue their claims, contrary to our obligations under international and domestic law,” the lawmakers wrote.The Biden administration has defended keeping the rule in place. “We continue to defer to the C.D.C. on its use,” Vedant Patel, a White House spokesman, said Wednesday, referring to the Centers for Disease Control and Prevention. Earlier this week, 33 House Democrats asked the C.D.C. director, Dr. Rochelle P. Walensky, to provide them with detailed explanations for why the public health rule was needed, particularly at a stage of the pandemic when vaccinations were widely available and new cases were declining.Several of the lawmakers on Wednesday suggested that the use of the public health justification was in conflict with the president’s agenda on promoting racial justice.“As the Biden administration works to fulfill its promise of racial equity, it must reverse Trump-era policies like Title 42 expulsions and conduct a full review of its treatment of Black migrants,” Rep. Bush said in a statement to The New York Times. She and Senator Booker, Democrat of New Jersey, initiated the letter and have been courting co-signers for months.Nongovernment organizations have written reports on the disparate treatment of Black migrants in the immigration system. They have found that Black migrants spend more time in solitary confinement while in immigration detention; are deported more often than any other race and are given higher bonds set by immigration judges.