In a stinging rebuke, the Democratic-controlled House passed legislation to terminate the emergency Trump declared at the US-Mexico border but Trump issued the first veto of his presidency in March to block that measure.
Morgan, who spoke to Congress on Thursday morning, said that many legislators at his hearing accepted that there was a crisis at the southern border with Mexico.
Trump warned last Friday that he would close the USA border with Mexico this week unless Mexico took action to help stop the flow of illegal migrants across the frontier.
Trump invoked national emergency powers in February, having tried to get wall funding from Congress without success.
Pelosi said in a statement Thursday that Trump was "stealing from appropriated funds". "No one is above the law or the Constitution, not even the President". "We must", he said.
US Attorney General William Barr said the president's emergency declaration was legal.
Abandoning his threat to immediately seal the southern border, President Donald Trump warned instead on Thursday that he'd slap tariffs on cars coming to the US from Mexico unless the country does more to stop the flow of migrants and drugs coming to the U.S. "The whole ballgame is cars", Trump told reporters.
In recent days, Trump has appeared to put his threat on hold, praising the Mexican government for doing more to apprehend migrants traveling through the country from Central America.
"The USMCA is a great deal for everybody, but this is more important to me than the USMCA", he said.
Trump plans to visit the border at Calexico, California, on Friday. However, Mr. Trump said that he cares more about USA security than about the trade agreement. The Council of Economic Advisers, she said, was conducting a number of studies on the impact, and "working with the president to give him those options".
Fearing that a border lockdown would prevent Mexico from shipping the 80 percent of avocados that the United States consumes, processors and wholesalers have started stockpiling the prized fruit used in guacamole or as spread on toast.
Mexican exporters are looking into sending their goods to the United States by air freight to avoid a five-mile-long line of trucks at the border caused by the Trump administration moving federal agents away from customs checks to immigration duties.