Salvador Allende won a narrow victory in a three way race in the 1970 Presidential election in Chile (the legislature officially voted him in as he was the leading vote getter, as was traditional). What if Chile had a runoff provision then, as it now does? The second place finisher, Allesandri was running as a moderate rightist, and the third place finisher, Tomic, was from the left of the Christian Democrats.
We know that in the 1972 congressional elections, held two years into Allende's reign, that a united Christian Democratic and conservative party alliance won about 56% of the vote, but that was after much polarization had taken place, in 1970 this hadn't happened yet, so Allende would have won a significant chunk of Tomic's voters, but I wonder if Allesandri (who was only narrowly behind Allende) would have come out ahead in a runoff.
I think Allesandri almost certainly would have won. Allende's alliance was going to have a mountain to climb to get to 50. Even if Tomic's voters were left wing on policy, voting for an alliance including the Communist Party was going to be hard to swallow.
