For months, presidential candidates have subjected themselves to relentless stumping, fundraising and vicious public scrutiny. They've endured far too many fact-checks and swing-state delicacies, and kissed far too many infants. Even with the exit polls now in sight, it's enough to make any sane person wonder: Why would anyone run for president?

On the record, our candidates cite similar reasons: that this is the most important election in the past however many years - and that they, however unworthy, have the right ideas to make a difference.

But what about the real reasons someone might decide to run? To find that kind of honesty, we must dig deep.

Presidents run because they care about their legacies. John Quincy Adams had already served several presidents as ambassador or secretary of State. In his private diary, he made only one mention of his motivation.

"Whether I ought to wish for success is among the greatest uncertainties of the election. Were it possible to look with philosophical indifference to the event, that is the temper of mind to which I should aspire. ... (But) to suffer without feeling is not in human nature; and when I consider that to me alone, of all the candidates before the nation, failure of success would be equivalent to a vote of censure by the nation upon my past service, I cannot dissemble to myself that I have more at stake upon the result than any other individual in the Union."

Presidents run because they obsess over a particular issue. As the election of 1860 approached, Abraham Lincoln and his allies used letters to strategize. While Lincoln never explicitly said why he decided to run, we can infer at least one reason from the letters he wrote after losing the 1858 Senate election to his famous debating partner, Stephen Douglas.

"The fight must go on. The cause of civil liberty must not be surrendered at the end of one, or even, one hundred defeats. Douglas had the ingenuity to be supported in the late contest both as the best means to break down, and to uphold the Slave interest. No ingenuity can keep those antagonistic elements in harmony long.... I am glad I made the late race. It gave me a hearing on the great and durable question of the age … (and) I believe I have made some marks which will tell for the cause of civil liberty long after I am gone."

Presidents run because they want power. Political watchers love to speculate on a candidate's motives, something John F. Kennedy understood. Did he end up in politics to please his father, or to measure up to his dead brother? For Kennedy, the answer was simpler, as he revealed to a friendly reporter shortly after announcing his run in 1960.

"Well, look now, if (I) went to law school … and then I go and become a member of a big firm, and I'm dealing with some dead, deceased man's estate, or I'm perhaps fighting in a divorce case … or let's say more serious work, when you're participating in a case against the DuPont Company in a general antitrust case, which takes two or three years, can you tell me that that compares in interest with being a member of Congress in trying to write a labor bill, or trying to make a speech on foreign policy? I just think that there's no comparison. ... Most important is the fact that the president today is the seat of all power."

Presidents run because they want to see an agenda through. Many of our most respected presidents were vice presidents first, including John Adams, Theodore Roosevelt and Harry S. Truman, who moved into the Oval Office when Franklin D. Roosevelt died in 1945. In the second volume of his "Memoirs," Truman wrote about this sense of obligation.

"If I had heeded the desire of my family, I would have made plans to leave the White House at the end of my first term. ... I had already been President of the United States for more than three and a half years. The compelling motive in my decision to run for the presidency in 1948 was the same as it had been in 1944. There was still "unfinished business" confronting the most successful fifteen years of Democratic administration in the history of the country. The hard-earned reforms of the years since 1933 which insured a better life for more people in every walk of American life were taking permanent root in the 1940's. These benefits were still vulnerable to political attack by reactionaries and could be lost if not safeguarded by a vigilant Democratic administration.

History can't completely answer the question of why anyone would want to be president. But one thing is certain: It's never an easy decision.

Craig Fehrman is working on a book about presidents and their books. He wrote this for the Los Angeles Times.

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