But Mr. Trump has steadfastly tried to deflect blame for the separation of children from their parents, consistently dissembling about why it is occurring. His comments are the latest example of his asking the public to discount what it sees with its own eyes and instead believe his own self-serving version of reality. They also reflect how politically poisonous the issue has become, as photographs and news articles circulate about the effects of the practice.

“I hate the children being taken away,” Mr. Trump told reporters on Friday in front of the White House during a 45-minute impromptu question-and-answer session on a wide range of topics. “The Democrats have to change their law — that’s their law.”

In fact, there is no law that requires families to be separated at the border. There is a law against “improper entry” at the border, as well as a consent decree known as the Flores settlement that limits to 20 days the amount of time that migrant children may be held in immigration detention, which a federal judge ruled in 2016 also applies to families. A 2008 anti-trafficking statute — signed into law by a Republican president, George W. Bush — also requires that certain unaccompanied alien minors be transferred out of immigration detention in 72 hours. None of those laws or precedents mean that children must be taken away from their parents.

It is the Trump administration’s decision this year to prosecute all unlawful immigrants as criminals that has forced the breakup of families; the children are removed when the parents are taken into federal custody. While previous administrations have made exceptions to such prosecutions for adults traveling with their minor children, the Trump administration has said it will not do so.

An official from the Department of Homeland Security who insisted on anonymity to discuss the policy said on Friday that there are, in fact, exceptions for babies, but could not provide an age cutoff above which a child may be taken from his or her parent. Data reviewed by The New York Times in April indicated that of more than 700 children separated from their parents since October, over 100 had been under the age of 4.