1) Bill Clinton. After all, it takes a cool guy to know a cool guy. "Obviously Bill Clinton," says Julian Zelizer, a presidential historian at Princeton University. During the 1992 election, Clinton explored new ways to reach voters, like appearing on MTV and playing the saxophone on a late night talk show. Clinton's popularity, Zelizer says, "even in the middle of impeachment, demonstrated a kind of admiration many had for his personal style."

2) John F. Kennedy. The smoothie from Massachusetts "was certainly cool in terms of charisma and demeanor," Zelizer says. The stark contrast between Kennedy and Richard Nixon in the 1960 debates "might have set the standard for what it meant to be cool."...