Brownsville (CNN) The first thing everyone notices when they arrive at the Rio Grande Valley is the heat.

While dozens of protestors stood outside a Customs and Border Protection processing detention center in McAllen, Texas, in more than 100-degree weather, the officials inside the facility told reporters during their tour that the temperature inside was about 72 degrees.

Democratic Sen. Jeff Merkley of Oregon, who first visited the South Texas border on June 3, described his second visit as more coordinated for Customs and Border Protection officials who gave the tour.

"We did see the children who were held inside here," he told CNN's Ana Cabrera in an interview. "In wire-mesh, chain linked cages that are about 30x30, a lot of young folks put into them. I must say though, far fewer than I was here two weeks ago. I was told that buses full (of children) were taken away before I arrived. That was one of my concerns, that essentially, when you have to give lengthy notice, you end up a little bit of a show rather than seeing what's really going on in these centers."

The heat outside was most noticeable during the press conference with visiting Democratic lawmakers who came to learn more about the facilities, as they stood, visibly sweating while talking to the assembled media.

This visit was the first experience for several lawmakers who traveled to McAllen and Brownsville to tour facilities where the Department of Homeland Security is processing and detaining immigrants illegally crossing the border.

The lawmakers came to South Texas, they say, to learn more about the agency's processing of undocumented immigrants entering the United States, including a policy to refer all people who cross the border illegally for criminal prosecution on top of immigration proceedings.

As a result of enforcing that policy, families who cross illegally have been separated from their children because those accompanying the children are put into the criminal justice system.

Customs and Border Protection officials offered reporters a tour of the McAllen facility before the lawmakers arrived. Inside, more than 1,100 immigrants, including children, were waiting to be processed. The warehouse-like facility has chain-link fences on the inside separating the immigrants. Mattresses with mylar blankets were strewn on the floors. Federal officials said the immigrants were only being processed and wouldn't be here longer than three days, but lawmakers reported they'd been told by immigrants within the facility that they had been there for seven.

While Democratic lawmakers on Capitol Hill are becoming increasingly vocal in criticizing the Trump administration's policy of "zero tolerance" when processing undocumented immigrants, White House press secretary Sarah Sanders argued Thursday that it's "biblical" for the Trump administration to enforce federal law in a way that separates children from their families at the border when they illegally enter the US.

Sanders' comments came when she was pressed by CNN's Jim Acosta on whether she agreed with Attorney General Jeff Sessions' assertion, citing Romans 13, that the Bible requires the Trump administration to follow the law.

Merkley told reporters during a press conference in Brownsville, "They call it 'zero tolerance,' but a better name for it is zero humanity, and there's zero logic to this policy."

The visiting group of Democrats included Merkley and Sen. Chris Van Hollen of Maryland, Reps. Sheila Jackson Lee of Texas, Peter Welch of Vermont, David Cicilline of Rhode Island and Mark Pocan of Wisconsin.

Former Housing and Urban Development administrator Julian Castro was also at the first stop of the tour, which was a processing center for immigrants -- where at one point he walked toward the side door of the facility holding stuffed animals and letters to deliver to the children in the facility.

The lawmakers also made stops at a Border Patrol facility, a port of entry from Mexico, an Immigration and Customs Enforcement facility and the facility noted as Casa Padre, which was converted from a Walmart into housing for unaccompanied minors and separated children.

Trump administration, Republicans respond to the policy

It's not just Democrats who have spoken out against the policy.

While the lawmakers were touring facilities in South Texas, Department of Homeland Security secretary Kirstjen Nielsen slammed reports from these lawmakers, advocacy groups and the media, saying immigrants are crossing the border and able to seek asylum.

"This misreporting by Members, press & advocacy groups must stop. It is irresponsible and unproductive. As I have said many times before, if you are seeking asylum for your family, there is no reason to break the law and illegally cross between ports of entry," she tweeted.

She ended the Twitter thread writing, "We do not have a policy of separating families at the border. Period."

However, Merkley said he heard multiple accounts from people who witnessed immigrants waiting at an international bridge to enter a US facility so they could seek asylum. This time, however, he said when they visited, he didn't see anyone waiting.

He told reporters when he asked where the people seeking asylum where staying or whether they'd all been processed, he couldn't get a definitive answer.

"The administration is arguing that by inflicting this harm, on children and parents, this stress, they're sending a message, a deterrent message, for people not to seek asylum in the United States," he said. "What they're doing is making it very difficult for people seeking asylum to actually cross at the legal border points."

And Pocan blasted Nielsen's tweets on the issue of separating kids from their parents.

"Donald Trump made this decision. He basically admitted at the end of last week he did this for leverage. It's a typical Donald Trump, inhumane, win-at-all-costs measure," he said.

Over the weekend, House Speaker Paul Ryan received backlash for what critics called a tone-deaf tweet about being a father to his family on the holiday.

The most notable critic was musician John Legend, who tweeted at him: "Seriously, f--- you. Reunite the families at the border and we can talk about father's day."

What Congress can do

In Congress, Senate Judiciary Committee ranking member Dianne Feinstein introduced a bill dubbed the "Keep Families Together Act" that would prevent the separation of immigrant children from their parents.

Democratic Rep. Jerry Nadler, the ranking member on the House Judiciary Committee, is planning to introduce the companion bill, a source told CNN. On Sunday, he tried to enter an ICE Detention Center in Elizabeth, New Jersey, alongside a group of Democratic lawmakers, where they had to wait for two hours before they were finally allowed to go inside.

"What we saw here today was heartbreaking," the New York Democrat said in a statement after the event. "These are people trying to escape extreme violence and poverty. What is happening to them is despicable. It is a sin."

He continued, "Trump and Sessions say they are following laws. That is a lie. There are no laws requiring families to be ripped apart. Trump claims Democrats are to blame for families being broken up. That is a lie."

The White House, however, has signaled it's using separating children from their parents to negotiate with Democrats on the issue of immigration.