By David Morgan and Makini Brice
WASHINGTON (Reuters) -Bipartisan negotiators in the U.S. Senate will release as early as Friday a long-sought - and politically charged - agreement to stem the flow of migrants across the U.S.-Mexico border, setting up a vote next week on a bill that would also provide fresh aid to Ukraine and Israel.
But the legislation already faces substantial opposition from Republicans in the Senate and the House of Representatives who are aligned with Donald Trump, the frontrunner for the Republican presidential nomination.
Senator Kyrsten Sinema, one of three bipartisan negotiators who have been working toward a border deal for months, said the emerging agreement will end the practice of releasing migrants caught crossing the border illegally, provide emergency authority to shut the border down and revamp the U.S. asylum system.
"We are changing the policy dramatically," the U.S. senator from Arizona, an independent, told reporters.
Trump has called on lawmakers to reject any deal ahead of the November elections that will determine control of the White House and Congress.
But Senate Majority Leader Chuck Schumer, the top Democrat in Congress, announced that an initial Senate vote on the supplemental aid package will take place no later than next Wednesday.
"We cannot simply shirk from our responsibilities just because the task is difficult," Schumer said on the Senate floor. "These challenges at the border and Ukraine and the Middle East are just too great."
Schumer said the text of the bill would be released no later than Sunday.
President Joe Biden asked Congress in October to approve a $106 billion emergency spending bill, including $61 billion for Ukraine as it battles Russian invaders and $14 billion for Israel in the aftermath of the Oct. 7 raids by Hamas. That request has been stalled by Republicans' insistence that it be tied to an unrelated shift in immigration policy.
The U.S.-Mexico border is a top issue for Republicans, with record numbers of migrants caught illegally crossing into the United States since Biden, a Democrat, took office in 2021.
The U.S. Border Patrol arrested about 2 million migrants at the border in fiscal-year 2023, similar to record-breaking totals during Biden's first two years in office.
Immigration also ranks as the second-greatest worry for Americans, according a Reuters/Ipsos poll published on Wednesday. Some 17% of respondents said it was their top concern, a sharp increase from December.
Schumer's announcement came a day after House Speaker Mike Johnson, the top Republican in Congress, cast doubt the future of any Senate border agreement, saying Biden does not need new laws to tackle the problem.
"From what we've heard, this so-called deal does not include transformational policy changes that are needed to actually stop the border catastrophe," Johnson said.
But some Republicans take issue with fellow party members who have attacked the legislation without knowing what is in it.
"There are some in the Senate and in the House who are desperately trying to sabotage it," Representative Dan Crenshaw, a Republican from Texas, told reporters, suggesting that blind Republican opposition was mainly about not helping Biden escape the issue in the November election.
"They're making it seem like the rest of us are against the bill. But that's just not true," Crenshaw added. "I want to secure the border. That's what I told my voters I would do."
(Reporting by Makini Brice and David Morgan in WashingtonAdditional reporting by Katharine Jackson in WashingtonEditing by Scott Malone and Matthew Lewis)