Catarina Gomez Lucas shows a picture of her 8-year-old brother, Felipe Gomez Alonzo, who died Dec. 24 while in the custody of US Customs and Border Protection officials.

ALAMOGORDO, NEW MEXICO — House Judiciary Committee chair Jerry Nadler on Monday bluntly warned President Trump against declaring a national emergency along the southern border and vowed to use Congress and the courts to challenge any such effort.

“We will oppose any effort by the president to make himself a king and a tyrant,” Nadler said.

President Trump on Tuesday will speak to the nation in a primetime telecast that has been billed as addressing a “national security” crisis at the border, and on Monday, Vice President Mike Pence told reporters that the White House is actively exploring the use of emergency powers to build a border wall.

“The president has no authority to usurp Congress’s power of the purse,” Nadler, of New York, told reporters here following a tour of a Customs and Border Protection facility in this remote town where 8-year-old Felipe Gomez Alonzo died last month. Felipe had been detained with his father after arriving from their village in Guatemala.

Rep. Joaquin Castro of Texas, the chair of the Congressional Hispanic Caucus, agreed, arguing it would be “profoundly inappropriate for the President of the United States to circumvent the legislative branch and single-handedly, against the will of the American people and the American Congress, put up a wall.”

Nadler said there is no national security crisis at the border, and that Trump is wrong to say there is one. “I expect the President to lie to the American people,” Nadler said, arguing, “the only emergency on the border is a humanitarian one caused by this administration’s war on children.”

A congressional source who was on the tour told BuzzFeed News that CBP officials acknowledged detention numbers were down. The source said that the officials told lawmakers that after months of detaining families along the New Mexico border, “the numbers are way down.” No detainees were being held at the Alamogordo facility as of Monday, and according to a source with Annunciation House, an El Paso–based nonprofit that provides housing to migrants released from CBP custody, there has been a “significant drop in releases to us.”