Many people are offering John McCain advice about a running mate. Various candidates would purportedly tip swing states into the McCain column, or improve his appeal to women and minorities. Maybe. LBJ did help JFK win Texas in 1960. But aside from that one case, it is hard to think of any vice presidential nominees who were strong assets. It is much easier to think of liabilities. So the governing maxim is: “Above all, do no harm.” McCain should pick someone who could handle the presidency and would not cause embarrassment. It would be nice if that person could also add some extra votes, but the key is to avoid damage.
Richard Nixon’s selections illustrate the point.

In 1968, he picked Governor Spiro Agnew of Maryland. As he explained in his memoirs, he needed to win the Midwest and Rim South. “Agnew fit the bill geographically, and as a political moderate he fit it philosophically.” Though he had once backed Rockefeller, his tough reaction to urban disorder made him acceptable to conservatives. So far, so good. But his qualifications were thin and his national experience nonexistent. Once the campaign started, he turned into a gaffe machine. After someone asked why he did not campaign in the inner city, he said: “When you’ve seen one slum, you’ve seen them all.”

A Hubert Humphrey TV spot consisted of the legend “Agnew for Vice President?”—and half a minute of laughter.

Agnew probably cost Nixon the votes of swing-state moderates. Some claim that he helped in the South, but the electoral map says otherwise. Between 1960 and 1968, the only Southern states that switched to Nixon were the Carolinas. North Carolina had long been trending Republican in presidential races. In South Carolina, the key was not the candidacy of Spiro Agnew but the support of Strom Thurmond.

Agnew later proved to be a catastrophe when he had to resign the vice presidency in a bribery scandal.

Eight years earlier, Nixon chose United Nations Ambassador Henry Cabot Lodge. As a diplomat and former senator, Lodge had the right background. But as Nixon biographer Stephen E. Ambrose observed: “That Lodge was a drag on the ticket there can be little doubt. Of the four candidates [the others being RN, LBJ, and JFK], he was the least energetic, made the fewest speeches, and drew the smallest number of voters.” He was also a patrician loose cannon, making an unauthorized promise than Nixon would name the first black cabinet member.

The moral is that McCain must avoid Agnew-like land mines and Lodge-like duds. First, his campaign should check for potential scandals. That process has become standard in recent decades. Second, advisers should study the political records of potential running mates, checking how they’ve fared in campaign speeches and debates. The second criterion would rule out nonpoliticians such as General Petraeus or Secretary Rice.

Again, McCain should remember a simple rule of thumb: a good running mate might help a little but a bad one could hurt a lot.