Texas Sen. Ted Cruz was born in 1970, six years after events refuted a theory on which he is wagering his candidacy. The 1964 theory was that many millions of conservatives abstained from voting because the GOP did not nominate sufficiently deep-dyed conservatives. So if in 1964 the party would choose someone like Arizona Sen. Barry Goldwater, hitherto dormant conservatives would join the electorate in numbers sufficient for victory. This theory was slain by a fact — actually, 15,951,378 facts. That was the difference between the 43,129,566 votes President Lyndon Johnson received and the 27,178,188 that Goldwater got in winning...