Republicans usually have a very set succession for presidential nominees. If you are the incumbent, you may be challenged in the primaries, but you will be renominated (Nixon '72, Ford '76, Reagan '84, G.H.W. Bush '92, G.W. Bush '04). If you are the incumbent vice president when the president is termed out, you will be nominated (Nixon '60, G.H.W. Bush '88). If the field is wide open (except for Goldwater '64), and you are the highest elected official, most persistent presidential candidate or political dynasty heir who is credible, you will be nominated (Nixon '68, Reagan '80, Bob Dole '96, G.W. Bush '00, McCain '08, Romney '12).
It's also helped to be a Nixon, Bush or Dole. Every Republican ticket since 1952 has had a representative of one of these families, except Goldwater / Miller and McCain / Palin (both losers, and both headed by Arizona senators--coincidentally?), though that era seems largely concluded.
So who can do it in 2012? We'll see if it's Mitt Romney and Paul Ryan.
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