31/100 of all voters are Democrats.
Pick the first person and the probability of selecting a Democrat is 31/100. Then the voter is tossed back into the pool of possible candidates for selection. You pick the second voter and the probability is again 31/100. Because you only have to be concerned with the cases in which you picked a Democrat the first time, you multiply the likelihood of getting second Democrat (31/100) by the proportion of the time you selected the first Democrat (31/100)
so, 31/100 x 31/100 = 961/10,000= .0961 or .096 (rounded)