The year is 1800. Americans go to the polls to elect a President. Which Founder do you favor? The Federalist incumbent, sixty-four-year-old John Adams, or the Republican challenger, fifty-seven-year-old Thomas Jefferson, who, awkwardly enough, is currently serving as Adams’s Vice-President?

Consider your vote carefully. This is the most important election in American history. What Jefferson dubbed “the revolution of 1800” marked the first transition of power from one party to another. It led to the passage, in 1804, of the Twelfth Amendment, separating the election of Presidents and Vice-Presidents. (Before that, whoever placed second became the Vice-President, which is what happened to Jefferson in 1796.) It might have—and should have—spelled the end of the Electoral College. At the time, many people, not all of them members of the Adams family, thought that it might spell the end of the American experiment. As Edward J. Larson observes in his new book, “A Magnificent Catastrophe: The Tumultuous Election of 1800, America’s First Presidential Campaign” (Free Press; $27), “Partisans worried that it might be the young republic’s last.”

To size up the candidates, what you need, for starters, is the word on the street—or, since the United States in 1800 is an agrarian nation, the word on the cow path. Adams: a Harvard graduate and Massachusetts lawyer who helped negotiate the Treaty of Paris in 1783 and served two terms as Washington’s Vice-President before his election to the Presidency in 1796. Distinguished, disputatious, short, ugly, hot-tempered, upstanding, provincial, learned (president of the American Academy of Arts and Sciences). Very clever wife. Suspected of wanting to be king. Loves England. Thinks his diplomats have to tread carefully with Napoleon. Signed into law the Sedition Act in 1798; depending on your point of view, this was either so that he could have anyone who disagreed with him thrown in jail or so that he could protect the country from dangerous anarchists.

Jefferson: former governor of Virginia, onetime Ambassador to France, Washington’s Secretary of State. Eminent, brilliant (president of the American Philosophical Society), surpassing prose stylist, author of the Declaration of Independence (with help from Adams), unrivalled champion of liberty, slave owner, grieving widower, rumored to have fathered children by one of his slaves. Tall, humorless, moody, zealous, cosmopolitan. Artistic. Loves France, not so worried about Bonaparte. Ardently opposes the Sedition Act. Reputed atheist.

Are you still on the fence? You’re out of luck: there will be no Presidential debates, and precious few speeches. (In 1800, Americans considered politicians’ putting themselves so far forward to be unforgivably tacky.) No campaign managers, no Web sites, no television ads, no YouTube interviews, not so much as a Horse and Cart Across America tour. When Adams took a roundabout route through Pennsylvania and Maryland on a ride from Massachusetts to the nation’s new capital city, one Jeffersonian newspaper editor asked, “Why must the President go fifty miles out of his way to make a trip to Washington?” ...