The MBTI is a widely used tool in the counseling and consulting professions. It was devised in 1943 and has been championed by Dr. David Keirsey who has his own similar tool called the Keirsey Temperament Sorter II. Esentially each of us are one of sixteen basic types of people based on our temperament (aka: personality).
Spoiler alert: If you take the Keirsey Temperament Sorter II now, you might want to come back to see how the presidential candidates compare to your type.
Also, a BIG disclaimer, the MBTI is not considered, psychometrically, a highly valid, reliable instrument to use as the predictive validity is low. However, it is very helpful/useful as a tool for self-knowledge and organizational development.
According to Emily Yoffe, the author of the article I mentioned, Hillary Clinton is an ESTJ (Extrovert, Sensing, Thinking, Judging type).
Hillary Clinton is a Guardian,... what Keirsey calls "the Supervisor." Supervisors are, Keirsey says, steadfast, cautious, methodical. They are the reliable, detail-oriented people without whom organizations and society fall apart—which is something ESTJs won't hesitate to point out. "[T]heir first instinct is to take charge and tell others what to do," says Keirsey. They are "devoted public servants, seeing their role in government … in almost sacred terms of self-sacrifice and service to others." This service is an obligation, not given "freely and joyously." As columnist Richard Cohen observed about Hillary, "Whether she meant to or not, she has presented herself as a model of caution, of experience hard-earned and not enjoyed. …"
Barack Obama, on the other hand is an ENFP (Extrovert, Intuitive, Feeling, Perceiving type).
Barack Obama—no one will be surprised to learn—is an Idealist... what Keirsey calls "the Champion." ENFPs, says Keirsey, are "filled with conviction that they can easily motivate those around them." Champions work to "kindle, to rouse, to encourage, even to inspire those close to them with their enthusiasm." Idealists "usually have a tongue of silver" and are "gifted in seeing the possibilities" of institutions and people. Here's Obama on leadership: "[W]e need leaders to inspire us. Some are thinking about our constraints, and others are thinking about limitless possibility."
John McCain is an ESTP (Extrovert, Sensing, Thinking, Perceiving type).
John McCain is an Artisan... what Keirsey calls the Promoter. The ESTP is, according to Keirsey, "practical, optimistic, cynical, and focused on the here and now." If the ESTP portrait gives you a feeling of déjà vu, it's because George W. Bush is an ESTP, too. They are a common presidential type: Both Roosevelts, JFK, and LBJ were ESTPs. "Artisans need to be potent, to be felt as a strong presence and they want to affect the course of events," writes Keirsey. They hunger to "have a piece of the action," "to make something happen" whether "on the battlefield" or "in the political arena." So many politicians are Artisans because "politics allows not only for maneuvering, excitement, and risk—but for powerful social impact."
Want to check out the track history of presidents based on type? Check here.
For another take, see what Personality Zone has to say.