WASHINGTON — When the Republican-controlled Congress first approved its tax bill in December, most Democrats believed it would be a political loser for the GOP. Indeed, a New York Times poll found that just 37 percent of Americans approved of the plan. To pass a bill of tax cuts and have it be so unpopular with the American people is an amazing achievement for the Republicans — it's never been done before, Senate Minority Leader Charles E. Schumer, D-N.Y., crowed.