Creamy Horseradish Potato Salad

1/2 cup mayonnaise
1/3 cup sour cream
3 tablespoons bottled white horseradish (not drained)
1 tablespoon white-wine vinegar
1 teaspoon sugar
3/4 teaspoon salt
1/2 teaspoon black pepper
1/3 cup chopped fresh chives
3 pounds cooked small (2-inch) boiling potatoes, cooled to room temperature and quartered

Whisk together mayonnaise, sour cream, horseradish, vinegar, sugar, salt, and pepper in a large bowl until smooth. Add chives and potatoes and stir to coat.