1½ cups amber rum (such as Appleton Estate)
1 cup Campari
1 cup sweet vermouth
1–2 750-ml bottles chilled sparkling wine
12 orange twists (optional)
Combine rum, Campari, and vermouth in a large pitcher; chill until very cold, at least 2 hours and up to 1 day.
For each cocktail, pour 2 oz. rum mixture into a Collins glass filled with ice and top off with 2–3 oz. chilled sparkling wine. Garnish with an orange twist, if desired.