Ruy Teixeira: “Start with this: when Democrats persuade a voter to switch sides, that nets two votes for the Democrats (one less for the Republicans, one more for the Democrats). When Democrats turn out one more voter to vote Democratic that is, of course, a net of only one vote for them.”
“But it’s really worse than that. Typically, Democrats think of increased base turnout in terms of turning out more voters from various pro-Democratic demographic groups—young voters, black voters, Hispanic voters, college-educated whites, whatever. But not all the voters in these groups favor the Democrats so mobilization of more voters from a given group may well net less than one vote per additional voter. For example, looking at current Congressional ballot preferences, Democrats might net only a third of a vote for every additional Hispanic or young voter, six-tenths of a vote from every additional black voter and just a sixth of a vote from every additional college-educated white voter.”
“The math looks even more unfavorable when the following is considered: Democrats tend to assume that nonvoters from a given demographic are the same politically as voters from the same group… except they don’t vote. But a mountain of political science evidence shows that’s not so.”
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