House Speaker John A. Boehner (R-Ohio) raised doubts Thursday that Congress will be able to approve President Obama's emergency funding request to address the growing influx of illegal migrants crossing the U.S.-Mexico border before lawmakers leave Washington for their summer recess.

Asked about the prospect of approving Obama's $3.7 billion request before the five-week break begins Aug. 1, Boehner said, "I would certainly hope so, but I don't have as much optimism as I would like to have."

Boehner said that no decision has been made yet by his leadership team on how to proceed on the issue. A "border working group" of Republican lawmakers is due to give Boehner a set of policy recommendations on how to deal with the crisis, but has not yet said when they will deliver their proposals.

Asked why he has less optimism than before that lawmakers will be able to move quickly on the subject, Boehner said, "There's just been some comments made by our colleagues across the aisle that are going to make this much more difficult to deal with."

That was a reference to growing opposition among Democrats to changing a 2008 law that grants extra protections to youths from Central American countries who cross the U.S.-Mexico border. Republicans and some Democrats have said it will be necessary to tweak the law in order to fulfill Obama's request for emergency funding.

"I don't know how Congress can send more money to the border to begin to mitigate the problem if you don't do something about the '08 law that is being abused. And it's being abused," Boehner told reporters at his weekly press conference.

Shortly before Boehner spoke, the head of the Senate Foreign Relations Committee reiterated that it would be not acceptable to change the 2008 law.

Sen. Robert Menendez (D-N.J.) made his comments at the start of a committee hearing on the border crisis.

"If you flee 2,000 miles and you were told by the gangs join or die, if you’re raped and you flee 2,000 miles not to ever experience that tragic and traumatic set of circumstances, you don’t come with anything but the clothing on your back, and when you get here to the United States, you’re going to need a reasonable period of time to be able to produce the facts to make that case that doesn’t come with you," Menendez said. "I understand the desire to accelerate the process, but accelerating without due process is not acceptable."

Menendez and other members of the Congressional Hispanic Caucus met with Obama at the White House on Wednesday to convey their opposition to changing the law. During the meeting Obama told the group that he wanted “to find a way to ensure due process but also speed things up” in the processing of young migrants, according to another lawmaker in the room who asked not to be identified in order to frankly describe the president’s opinions.

House Minority Leader Nancy Pelosi (D-Calif.) and other Democratic lawmakers have said in recent days that they oppose changing the law.

During the Senate hearing, officials from the departments of Justice and State said that more than 50,000 unaccompanied children from Central American countries have been apprehended along the U.S.-Mexico so far this year. Nearly three-quarters are males between the ages of 15 and 17, the officials said.

Citing data showing a dramatic increase in illegal border crossings over the past two years, Republican senators pressed the witnesses on whether steps by the Obama administration -- including the 2012 decision allowing some illegal immigrants who came to the United States as children to get two-year work permits and a deferral of deportation proceedings -- has driven the influx.

But the witnesses said that ongoing migrations north are due chiefly to raging drug-related violence that is fueling economic instability in El Salvador, Guatemala and Honduras.

Thomas A. Shannon, a State Department official with oversight of the border crisis, said the current political debate over immigration "does not have an impact" on the recent influx. He said that U.S. officials have always stressed to migrants that they will be deported if they illegally cross the border.

"Children have been deported and will be deported," he said.

Bruce Swartz, a deputy assistant attorney general responsible for prosecuting drug and immigration crimes, said that Central American drug cartels and human smugglers "are marketing misunderstandings about how U.S. immigration law will work."

Senators also sought information about conditions along the route from Central America, through Mexico to the United States. Sen. Marco Rubio (R-Fla.) asked Shannon whether it is true that local medical officials and human smugglers are advising women to use birth control or other contraception as they travel north because they face the risk of rape.

"Not just women, but girls," Shannon said.

Meanwhile, Sens. John McCain (R-Ariz.) and Jeff Flake (R-Ariz.) formally unveiled legislation Thursday that they say would help make it less attractive for would-be immigrants to travel north to the United States.

The proposal would require make the changes to the 2008 law sought by Republicans and require that young migrants apprehended at the border be held in federal custody until they are repatriated or officials determine that their status allows them to remain in the country.  The legislation also would authorize the temporary hiring of about 300 personnel to pare down a backlog of immigration cases; increase the number of refugee applications by up to 5,000 each for El Salvador, Guatemala and Honduras.; but require the Obama administration to certify that the countries are working to prevent a mass exodus of citizens or risk losing American foreign aid.

Sen. Ted Cruz (R-Tex.) was also planning Thursday to introduce a measure that would block the Obama administration from expanding the policy that allowed some children of illegal immigrants to stay in the country and obtain work permits or the deferral of deportation proceedings. Aides said that Cruz's proposal would only block the expansion of the Deferred Action for Childhood Arrivals program, but does nothing to address the status of those already benefiting from the program.

Cruz told reporters that he rejected the idea that the 2008 law caused the flood of children at the border, noting that the surge happened only recently, after the 2012 executive action by Obama. “What caused this crisis was President Obama in 2012,” he said, suggesting the “promise of amnesty” for children led to the Central American families to send their children to the U.S. border.

“Then the 2008 law had an unintended consequence,” he said, forbidding border patrol agents from immediately returning those children from Central America.

His legislation would rescind Obama’s 2012 order, something other Republicans support but acknowledged it did not deal with the children already here. “It sounds like a good idea, but it’s prospective,” Sen. John Cornyn (R-Tex.) said.