WASHINGTON (AP) — Democratic leaders in the House proposed tighter requirements for the care of unaccompanied refugee children as they sought to pass a $4.5 billion emergency funding bill to address the humanitarian crisis involving the thousands of migrant families detained after crossing the U.S.-Mexico border.

Lawmakers and aides said they expected the changes, which were concessions to Hispanic and liberal Democrats, to produce a winning tally when the measure comes to a vote later Tuesday. A full court press by leaders, including Speaker Nancy Pelosi, D-Calif., was also helping nail down support, though some Democrats had lingering reservations.

The Senate plans to vote on a different, and bipartisan, companion measure in coming days as the chambers race to wrap up the must-do legislation by the end of the week. The White House is threatening to veto the House bill, saying it would hamstring the administration's border security efforts, and the Senate's top Republican suggested Tuesday that the House should simply accept the Senate measure — which received only a single "nay" vote during a committee vote last week.

"The idea here is to get a (presidential) signature, so I think once we can get that out of the Senate, hopefully on a vote similar to the one in the Appropriations Committee, I'm hoping that the House will conclude that's the best way to get the problem solved, which can only happen with a signature," said Senate Majority Leader Mitch McConnell, R-Ky.

House Democrats seeking the changes met late Monday with Pelosi, and lawmakers emerging from a morning caucus meeting were supportive of the legislation. Hispanic lawmakers were still seeking additional changes, however.

In the closed-door meeting Tuesday morning, Pelosi urged them to rally strongly behind the legislation in hopes of increasing their leverage when they negotiate a compromise package with the Republican-run Senate. There was silence in the room when she asked if any lawmakers had problems with the legislation, according to a senior Democratic aide who spoke on condition of anonymity to describe the private session.

Pelosi also warned that President Donald Trump's hand would be strengthened if the legislation failed, the aide said.

"The president would love for this bill to go down today," she said, according to the aide. "A vote against this bill is a vote for Donald Trump and his inhumane, outside-the-circle-of-civilized attitude toward the children."

Changes unveiled Tuesday would require the Department of Homeland Security to establish new standards for care of unaccompanied immigrant children and a plan for ensuring adequate translators to assist migrants in their dealings with law enforcement.

Many children detained entering the U.S. from Mexico have been held under harsh conditions, and Customs and Border Protection Chief Operating Officer John Sanders told The Associated Press last week that children have died after being in the agency's care. He said Border Patrol stations are holding 15,000 people — more than triple their maximum capacity of 4,000.

Sanders announced Tuesday that he's stepping down next month amid outrage over his agency's treatment of detained migrant children.

Congress plans to leave Washington in a few days for a weeklong July 4 recess. Democratic leaders said they wanted the House and Senate to send Trump a final, compromise bill this week, rather than going home and facing constituents' accusations of not responding to the migrants' needs.

In a letter Monday threatening the veto, White House officials told lawmakers they objected that the House package lacked money for beds the federal Immigration and Customs Enforcement agency needs to let it detain more migrants. Officials also complained in the letter that the bill had no money to toughen border security, including funds for building Trump's proposed border wall.

"We've got lives at stake," said Rep. Tony Cardenas, D-Calif. He said the U.S. has been "the gold standard" for treating refugees fleeing dangerous countries, "and I don't think we should compromise that at all."

Much of the legislation's money would help care for migrants at a time when federal officials say their agencies have been overwhelmed by the influx of migrants and are running out of funds.

"This is strictly a supplemental that's in response to a humanitarian crisis that is taking place right now," said Rep. Lucille Roybal-Allard, D-Calif., one of the authors of the bill. She said language in the measure limits the use of the funding to "food, clothing, better shelter facilities and so on."

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AP Congressional Correspondent Lisa Mascaro and Associated Press writers Andrew Taylor and Colleen Long contributed to this report.