As context, McCain stated the central criterion in making his choice would be that the person was ready to assume the Presidency if needed.

This was a criterion repeatedly stated by McCain and his supporters. “He’ll pick a vice presidential nominee that he thinks will help the country if something happened to him, lead the country if something happened to him,” Sen. Lindey Graham, R-S.C. said in May.

Two out of the last seven U.S. presidents (or 28%) have not finished their terms; the vice president was required to fill the office for the remainder of the president’s term. A presidential vacancy can happen due to assassination, impeachment, natural death or accidental death (for example, car accident).

As U.S. News and World Report wrote in an in-depth examination of the presidential succession issue, 9 vice presidents have become president. In other words, 9 out of the 34 presidents who first became president by election had to be replaced, which is a rate of 26%. If you add the 9 accidental presidents to the 34 originally elected presidents, there have been 43 presidents in all. So 20% of American presidents were vice-presidents who had to step in.

For some, the fact that McCain is 72 and would be the oldest person ever elected president adds greater weight to his choice of vice president.

“By the time this election gets around, everyone is going to know he [would] be the oldest president ever sworn in,” Republican consultant Scott Reed said . “It’s a concern and it has to be addressed.”

McCain repeatedly states he always puts “country first.” At a candidates forum held by Saddleback Church televised on CNN last month, he used the phrase six times, including: “When I go to places where I know they probably won’t vote for me, I know that my job is to tell them that I’ll be the president of every American and I’ll always put my country first.”