It’s beginning to look like a showdown between ’70s icons Rhoda Morgenstern and Laura Ingalls. Valerie Harper, 61, best known as the star of TV’s “Rhoda,” will challenge Melissa Gilbert, 37, best known as the star of “Little House on the Prairie” for the presidency of the Screen Actors Guild in an election to be held this fall. The race to replace the current SAG president, former “St. Elsewhere” costar William Daniels, 74, is heating up, in that Daniels (whose second term expires in November) has endorsed Harper. “I think Valerie will make a great president because she is a centrist who can pull together all sides of the union — right and left, east and west,” Daniels told Daily Variety. “She is a dear friend, and I’m very happy with her decision to run.” Other early endorsements for Harper came from former SAG president Charlton Heston and Richard Dreyfuss, who earlier this year was the subject of speculation that he would run for the post. The actors union has some 98,000 members.