WASHINGTON — Last fall, Joseph R. Biden Jr. made a stop in Miami’s Little Haiti neighborhood during his campaign, pleading with residents to register to vote. “I’ll give you my word as a Biden, I’ll be there,” he told them. “I’ll stand with you.”

But as the Biden administration ignored pleas from advocates and some lawmakers not to deport thousands of Haitians back to their broken country last month after they arrived en masse in Del Rio, Texas, many Haitian Americans say the man they turned out for in last year’s election is nowhere to be found.

“People absolutely felt abandoned when they saw the horrific images in Del Rio,” said Vanessa Joseph, the chairwoman of the Haitian American Voter Empowerment Coalition.

The White House responded with outrage to viral images of Border Patrol agents on horseback, corralling Black migrants in scenes that some compared to the treatment of fugitive slaves. Some also made connections to police violence against Black Americans. Mr. Biden said the agents would “pay” for their actions and awaits the results of a continuing investigation.

The Border Patrol’s actions in Del Rio are expected to come up on Tuesday during the Senate confirmation hearing for Mr. Biden’s choice to lead Customs and Border Protection, Chris Magnus. Mr. Magnus, the police chief in Tucson, Ariz., has a reputation for changing the culture of law enforcement agencies.

But what happened in Del Rio is not just about the horses, immigration and civil rights advocates say. It is about the poor treatment of Black migrants that has spanned administrations, often overshadowed by the larger debate about the broken immigration system and a persistent focus on waves of migrants from Central America.

Recent reports have found that Black migrants are more often placed in solitary confinement in immigration detention; face a higher rate of deportation than any other race; and see higher bonds set by immigration judges.

Alejandro N. Mayorkas, the homeland security secretary, said at a news briefing last month that he “would respectfully disagree” that Black migrants received disparate treatment. He said that in Del Rio, policy was carried out “irrespective of the country of origin, irrespective of the race of the individual.”

Regardless, the images of agents on horseback drew such a visceral reaction not just from immigration advocates, but also from Black Americans and powerful civil rights organizations like the N.A.A.C.P., that many see this as a moment to effect change.

“The connections have been made for Black people,” said Judith Browne Dianis, the executive director of the Advancement Project, a civil rights organization. “And accountability requires that the Biden administration act, because all Black people saw it, and we can’t unsee it.”

Even after Mr. Biden condemned the corralling of migrants in Del Rio, his administration sent dozens more deportation flights to Haiti, nearly 8,000 people, according to the Haitian Bridge Alliance, an advocacy group. Many say this makes little sense for a country that the administration has described as having “a political crisis and human rights abuses, serious security concerns” and “a dire economic situation” because of the pandemic. On Saturday, gang members in Haiti kidnapped 17 missionaries.

The response to Del Rio is emblematic of the Biden administration’s approach to border security, which many immigration advocates have said looks nothing like the “humane” immigration system that Mr. Biden promised during last year’s campaign. And the mass deportation of Haitians to a crisis-torn country, advocates say, is all the more disappointing from a president who has prioritized racial equity.

The issue could become a political liability for Democrats going into the 2022 midterm elections, said Adrianne Shropshire, the executive director of BlackPAC, a political organizing group. Black voter turnout in 2022, she said, will be critical for the Democrats to hold onto their majorities in the House and the Senate.

“It would be a mistake to think that Black Americans are somehow not paying attention to immigration issues,” Ms. Shropshire said, “particularly because the images that we saw are so jarring.”

Over a little more than two weeks last month, about 28,000 migrants crossed the border illegally into Del Rio, coming in large groups across the Rio Grande and overwhelming the Border Patrol.

While officials initially thought most of the migrants were Haitians, recent internal data shared with The New York Times shows that was not the case. There were thousands of others from a wide range of countries including large numbers from Brazil, Nicaragua and Venezuela.

Even so, there were no deportation flights to Venezuela, because the United States is limited in where it can send direct flights based on relations and agreements with individual countries. And despite the number of migrants from Brazil and Nicaragua, there was no increase in the number of deportation flights sent there. The overwhelming majority of deportation flights went to Haiti, with others going to Cameroon, the Dominican Republic, Ecuador, Guatemala, Liberia, Mexico, Rwanda and Senegal, according to an administration official who was not authorized to speak publicly.

Democrats in Congress have also pushed the administration to temporarily stop deporting Cameroonians, citing armed conflicts in the country. The Trump administration was accused of mistreating people from the central African nation.

In testimony obtained by The Times, Mr. Magnus, Mr. Biden’s pick to lead Customs and Border Protection, is prepared to say on Tuesday, “I will also expect — without exception — that all agency personnel be conscientious, fair and humane when enforcing the law.”

The Biden administration has said that it does not racially discriminate in its immigration enforcement actions, and that the response to the Del Rio situation was consistent with its border policy during the pandemic, carried over from the Trump administration. A public health rule put in place at the beginning of the pandemic allows for migrants to be turned back at the border even if they are seeking asylum.

Yet even within the government, anger about the mass deportation of Haitians from Del Rio has boiled over. Daniel Foote, the senior American envoy for Haiti policy, resigned over the administration’s response to the migration surge in Del Rio. And Harold Koh, who recently left his job as a senior adviser at the department, circulated a memo criticizing the administration’s continued use of the public health rule, “especially as it affects Haitians.”

The administration extended temporary humanitarian relief for Haitians who were already in the country by late July, making good on that piece of Mr. Biden’s campaign pledge to Little Haiti a year ago. But many, including Mr. Koh, argue that conditions in Haiti — ravaged by natural disasters and fraught with civil upheaval after its president was assassinated over the summer — continue to be so poor that humanitarian relief should be extended even further.

“Simply put, Haiti is a humanitarian nightmare,” Mr. Koh wrote in the memo.

Senators on the Foreign Relations Committee recently sent a letter to the secretaries of state and homeland security to express “outrage and disappointment over the cruel treatment of Haitians at our border, and their summary deportations.”

And some progressive lawmakers have grown so dissatisfied with the president’s handling of immigration issues that they have begun taking extraordinary steps to try to hold the administration to account. After hearing conflicting reports from the White House and immigration advocates about what was happening in Del Rio, Representative Cori Bush, Democrat of Missouri, dispatched her chief of staff to scrutinize conditions there and across the Rio Grande in Mexico.

The investigation into the treatment of the migrants in Del Rio is being conducted by Customs and Border Protection’s Office of Professional Responsibility. But activists question how thorough it can be when witnesses and victims were deported to Haiti.

“They cleaned the crime scene,” said Nana Gyamfi, the executive director of the Black Alliance for Just Immigration.

Although it was unusual for such a large group of migrants to cross the border illegally at once, what happened in Del Rio is most likely just the tip of the iceberg. Currently, tens of thousands of migrants — including many Haitians — are crossing a dangerous and roadless jungle that connects South America to the north, fleeing economic hardship caused by the pandemic.

Mr. Mayorkas recently said that the Homeland Security Department was prepared for something similar to the Del Rio surge happening in the future.

But there is little indication that the policies used to turn Haitian and other migrants away in September will change, particularly as the administration fights in court to keep the Trump-era public health rule in place.

“Is the Biden administration ready to do something different when the next wave of Black migrants wind up at the border?” said Ms. Dianis, of the Advancement Project. “I am not confident that they have a different plan than the one that they used this time.”

Luke Broadwater contributed reporting.