Lightly grease a 6-cup gelatin mold or decorative cake mold. Pour 1 1/2 cups tangerine juice into a heatproof bowl; sprinkle with the gelatin. Let stand until softened, about 5 minutes. Microwave at high power until hot, 3 to 4 minutes.
Transfer the mixture to a large bowl and whisk in the remaining 1 1/2 cups tangerine juice and the champagne. Pour into the mold and refrigerate until set, 3 hours or up to 2 days. To unmold, wrap the mold in a hot-water-soaked towel and invert onto a large, chilled plate.