One-third of the electorate who describe themselves as "born-again" or evangelical Christians gives Vice President George Bush a crucial margin of support in early test elections against Colorado Sen. Gary Hart, the leading Democratic challenger for the presidency in 1988, according to the Gallup Poll. In combined results of two recent surveys, Bush was found to be the choice of 50% of registered evangelicals, to 40% for Hart, with 10% undecided. Bush also leads New York Gov. Mario M. Cuomo, 59% to 29%. However, among non-evangelicals, Hart holds a 48%-43% edge over Bush, with Cuomo still trailing Bush, 39% to 50%.