Jeh Johnson, former DHS secretary in Barack Obama's administration, called presidential candidate Julian Castro's proposal to decriminalize illegal border crossing essentially advocacy for "open borders."

"That is tantamount to declaring publicly that we have open borders," Johnson told the Washington Post. "That is unworkable, unwise and does not have the support of a majority of American people or the Congress, and if we had such a policy, instead of 100,000 apprehensions a month, it will be multiples of that."

Other 2020 candidates, including Sens. Elizabeth Warren (D., Mass.), Kirsten Gillibrand (D., N.Y.), and Kamala Harris (D., Calif.) as well as South Bend Mayor Pete Buttigieg have embraced the proposal. Eight of the more than 20 candidates have expressed explicit support for open borders. Sen. Cory Booker (D., N.J.) said he would "virtually eliminate immigration detention," if elected president.

The crisis at the United States' southern border reached an inflection point in May, when border agents took more than 144,000 migrants into custody: the highest one-month total in 13 years, according to the Washington Post. At the same time, immigration courts have faced a backlog of people facing asylum hearings as well as over-crowded detention centers across the southern border.

Numbers of immigrants seeking asylum in the United States dipped in June, the Post reports.

Castro has explained his plan for open borders as a reaction to what he sees as "cruelty" on the part of President Donald Trump's administration.

"It calls for us to offer a more strong, bold and compelling vision on immigration," he said of his plan, adding that "I’m confident there are a lot of Americans would agree with me" on open borders.

Castro denied that there was a crisis at the border in January.

"The president has said there is somehow this crisis at the border," Castro said. "That's a lie. That couldn't be further from the truth."