In Texas Borderland, Trump’s Immigration Push Suffers Its Worst Legal Defeat Yet

Judge Fernando Rodriguez Jr., a bespectacled, soft-spoken 56-year-old nominated by President Trump, turned his high-backed leather chair toward a government lawyer at the federal courthouse in Brownsville, Texas, and asked a question. Can the president define what counts as an invasion, then declare that an invasion is happening, and then use a 1798 war powers law to expel the so-called invaders?

“Yes,” answered Michael Velchik, a Justice Department lawyer.

Judge Rodriguez followed up: Wouldn’t that make Mr. Trump’s powers under the wartime law, the Alien Enemies Act, “effectively limitless?”

The question hinted at a groundbreaking ruling that Judge Rodriguez issued on Thursday when he found that Mr. Trump was wrong to claim that the activities of Tren de Aragua, a Venezuelan gang in the United States, amounted to an “invasion” that justified invoking the wartime law.

The decision was the most sweeping ruling issued so far by a federal judge blocking the most aggressive prong of Mr. Trump’s effort, one that was already used to deport nearly 140 Venezuelans to a prison in El Salvador on March 15. It comes after a Supreme Court decision in early April that Venezuelan detainees facing potential deportation under the Alien Enemies Act could file lawsuits in the district courts where they were being held.

The result of the court’s order has been that challenges to a key piece of Mr. Trump’s immigration agenda, which began in Washington, are spreading around the country, filling the dockets of federal judges and drawing tough and skeptical questioning — even from jurists with impeccable conservative credentials.