WASHINGTON — President Donald Trump is edging closer to declaring a national emergency to pay for his long-promised U.S.-Mexico border wall as pressure mounts to end the three-week impasse that has closed parts of the government and deprived hundreds of thousands of workers of their salaries. Some 800,000 federal employees, more than half still on the job, were due to miss their first paycheck Friday under a stoppage that neared a record for the longest government shutdown.