Hold 47% to 44% advantage in registered voters' preferences.PRINCETON, NJ -- Registered voters now say they prefer the Republican to the Democratic candidate in their district by 47% to 44% in the midterm congressional elections, the first time the GOP has led in 2010 election preferences since Gallup began weekly tracking of these in March. The March 22-28 results were obtained after the U.S. House's passage of landmark healthcare reform legislation on March 21. The shift toward Republicans raises the possibility that the healthcare bill had a slightly negative impact on the Democrats' political fortunes in the short run. A...