Why haven't the Republicans been able to win an election without Nixon or a Bush on the ticket since 1929?

After Hoover was elected in 1929, we did not have another Republican president until Ike was elected with Nixon as VP. After that we had Nixon/Ford, Reagan/Bush, Bush/Quayle, Bush/Cheney.

Trump needs to pick Jeb as his running mate. History says he cannot win without him.

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