(Bloomberg) -- Homeland Security Secretary Kirstjen Nielsen called on Congress to change laws she said force the government to separate immigrant children from parents who cross the U.S. border unlawfully and said it is “very cowardly” for Democrats to allege the Trump administration is using the controversy for political leverage.

Nielsen both defended President Donald Trump’s family separation policy at a White House briefing on Monday and maintained that it was not his creation, but rather a consequence of the government’s prosecution of immigrants apprehended illegally crossing the border.

“Parents who entered illegally are, by definition, criminals,” Nielsen said. “Here is the bottom line: DHS is no longer ignoring the law.”

Yet there is no law requiring children to be taken from parents apprehended after unlawfully crossing the border. The Trump administration adopted the policy in April.

She said that previous administrations had also at times separated children from immigrant parents at the border, though “their rate was less than ours.”

“But they absolutely did this. This is not new,” she said. The Trump administration, she said, was engaging in the practice more frequently because U.S. law forbids the government from detaining “family units” apprehended while crossing the border for longer than 20 days.

Her news conference punctuated a day in which President Donald Trump and Nielsen each struggled to defend the separation of immigrant children from their parents against mounting bipartisan criticism. In Texas, several Democratic members of Congress said at a news conference that they had toured one U.S. detention center and encountered children under the age of one who they were told had been in government custody for more than a month.

Nielsen described a crisis on the southwest U.S. border, where she said illegal crossings exceed 50,000 people a month -- “multiples over each month last year” -- and authorities have observed large increases in crossings by unaccompanied children and families. Asylum claims have surged, she said, creating a backlog of 600,000 cases.

CNN and Quinnipiac University each released polls on Monday showing that two-thirds of Americans oppose Trump’s “zero tolerance” policy for border crossings that has led to children being taken from their parents. But both organizations also found that a majority of Republicans support the policy.

ProPublica published a recording it said was made last week in a U.S. detention center in which 10 Central American children can be heard crying and screaming for their mothers and fathers. One unidentified Border Patrol agent remarks, “Well, we have an orchestra here. What’s missing is a conductor.”

Trump continued to blame Democrats, falsely, for a policy that even members of his own party said he could end with a single phone call.

The president said at a White House event on Monday that “all of the problems” the U.S. is having with immigration are “very strongly the Democrats’ fault.” He insisted again that his family separation policy was the consequence of “horrible laws” that the opposition party refused to agree to change.

“The United States will not be a migrant camp and it will not be a refugee holding facility,” Trump vowed. “You look at what’s happening in Europe, you look at what’s happening in other places; we can’t allow that to happen to the United States, not on my watch.”

After denying that the family separation policy existed in a tweet on Sunday, Nielsen defended it in a speech earlier on Monday in New Orleans and said border authorities “will not apologize for doing our job.” People seeking asylum in the U.S., she said, should go to official ports of entry instead of crossing the border illegally.

House Democratic Leader Nancy Pelosi and Senator Kamala Harris, a California Democrat, both called for Nielsen to resign after the speech.

The Department of Homeland Security issued a statement denying allegations by Democrats and immigrant advocates that it has made it difficult for people to apply for asylum at ports of entry. But it said that Customs and Border Patrol “must prioritize its limited resources to ensure its primary mission is being executed,” which it said does not include asylum petitions.

--With assistance from Jennifer Jacobs.

To contact the reporters on this story: Margaret Talev in Washington at mtalev@bloomberg.net;Jennifer Epstein in Washington at jepstein32@bloomberg.net

To contact the editors responsible for this story: Alex Wayne at awayne3@bloomberg.net, Mike Dorning

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