WASHINGTON — The White House is facing sharp condemnation from Democrats for its handling of the influx of Haitian migrants at the U.S. southern border, after images of U.S. Border Patrol agents on horseback using aggressive tactics went viral this week. Striking video of agents maneuvering their horses to forcibly block and move migrants attempting to cross the border has sparked resounding criticism from Democrats on Capitol Hill, who are calling on the Biden administration to end its use of a pandemic-era authority to deport migrants without giving them an opportunity to seek asylum in the United States. Senate Majority Leader Chuck Schumer, D-N.Y., an administration ally, said images of the treatment of the migrants “turn your stomach” and called on the administration to discontinue the “hateful and xenophobic” policies of Biden’s predecessor, Donald Trump. “The policies that are being enacted now — and the horrible treatment of these innocent people who have come to the border — must stop immediately,” Schumer told the Senate on Tuesday. At the same time, the administration continues to face attacks from Republicans, who say Biden isn’t doing enough to deal with what they call a “crisis” at the border. Reflecting the urgency of the political problem for the administration, Homeland Security chief Alejandro Mayorkas said Tuesday the images “horrified” him, a seeming shift in tone from a day earlier, when he and others were more sanguine about the situation at the border. Mayorkas announced later that the agents involved have been placed on administrative duties pending the outcome of an investigation.