Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix rhubarb with 2 cups sugar in a large bowl and spread into the prepared baking dish; sprinkle rhubarb with raisins.
Beat vegetable shortening with 2/3 cup sugar in a bowl; beat in eggs, one at a time, followed by vanilla extract. Whisk flour, baking powder, and salt together in another bowl. Gradually stir the flour mixture into the wet ingredients, alternating with milk, to form a smooth batter. Spread batter over fruit in baking dish.
Bake in the preheated oven until rhubarb filling is bubbling and topping is set and golden brown, about 1 hour.