Make the simple syrup: Bring the sugar and 1/3 cup water to a simmer in a small saucepan, stirring to dissolve the sugar. Remove from the heat and let cool completely, about 20 minutes.
Combine the white wine, grapefruit juice, Campari and simple syrup in a pitcher and refrigerate until ready to serve. Pour into ice-filled glasses and top with the seltzer. Garnish with raspberries and grapefruit.