Homeland Security Secretary Kirstjen Nielsen said the debate over President Trump's declaration of a national emergency at the southern border boils down to "pretty simple calculus."

Twelve Senate Republicans joined the Democrats' effort last week to block Trump's emergency declaration, including the party's 2012 presidential nominee Mitt Romney.

The resolution, which the president vetoed the following day, would have blocked Trump from seizing billions of dollars intended for other projects in order to pay for his long-promised border wall.

"Did the president have authority to issue the emergency? He did. It's statutory. Congress gave it to him," Nielsen said on "Special Report" Monday.

She said she can confirm there is absolutely a humanitarian disaster at the border, and it's becoming more "heart-wrenching" every single day.

"We truly are in system meltdown," Nielsen said. "The system was not built for this type of flow, for these numbers and what we're seeing, whether it's drugs, whether it's traffickers, whether it's the child exploiters, or whether it's all the families and the children who are coming across very sick and very in need."

She pointed to stunning statistics released by U.S. Customs and Border Protection, which revealed that in February, more than 76,000 people tried to cross the U.S.-Mexico border. The total was more than double the number from the year before.

She said she remains optimistic that Congress will address the loopholes and issues in U.S. immigration law, because it is more vital now than ever.

"We've moved beyond all blinking lights. Congress must act. The president has made this a priority, continues to make it a priority in every way he possibly can exercise his authority," Nielsen said, concluding that it comes down to fixing "Congress, courts and criminals."

Watch the "Special Report" interview above.

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