ReutersWASHINGTON U.S. President Donald Trump was poised on Friday to declare a national emergency at the U.S.-Mexico border, a move that Democrats vowed to challenge as an unconstitutional attempt to fund his proposed border wall without approval from Congress.Trump was also expected to sign a bipartisan government spending bill approved by Congress on Thursday that would prevent another federal shutdown by funding several agencies that otherwise would have closed on Saturday morning.The Republican president was scheduled to deliver remarks on the issue at 10 a.m. EST (1500 GMT) in the Rose Garden at the White House.The bill, lacking any money for his wall, is a defeat for Trump in Congress, where his demand for $5.7 billion in barrier funding yielded no results, other than a record-long 35-day December-January partial government shutdown that damaged the U.S. economy and his poll numbers.Reorienting his wall-funding quest toward a legally uncertain strategy based on declaring a national emergency could plunge Trump into a lengthy battle with Democrats and divide his fellow Republicans.Even before the White House said on Thursday that Trump would declare an emergency, Republican senators, while sympathetic to his view that the southern border is in crisis, were favor .Trump was expected to sign it and declare an emergency, then fly to his private golf club in Florida for a holiday weekend break.