WASHINGTON — President Joe Biden plans to lift his predecessor’s historically low cap on refugees by next month, after initially moving only to expand the eligibility criteria for resettlements and getting swift blowback from allies in return. In an emergency determination signed Friday, Biden stated the admission of up to 15,000 refugees set by President Donald Trump this year “remains justified by humanitarian concerns and is otherwise in the national interest.” But if the cap is reached before the end of the current budget year and the emergency refugee situation persists, then a presidential determination may be issued to raise the ceiling. That set off a deluge of criticism from top allies on Capitol Hill such as the second-ranking Senate Democrat, Dick Durbin of Illinois, who called that initial limit “unacceptable.” White House press secretary Jen Psaki said later that Biden is expected to increase the refugee cap by May 15, though she didn’t say by how much. Asked Saturday by reporters about the cap, Biden didn’t offer new details.