The most important election of a lifetime? So say Gingrich et al.

Newt Gingrich sometimes refers to 2012 as "the most important election since 1860," which set the stage for the Civil War. His Republican competitors are a bit less flourishing in their comparison, describing November as the vote of a lifetime (though Mr. Gingrich also uses that phrase). Across the political divide, House Democratic leader Nancy Pelosi agrees: It's "the most important election of our generation."

If such descriptions sound familiar, it's because Americans hear them every four years. "This is certainly the most important election in my lifetime – not just because I'm running," said Barack Obama to a Wisconsin crowd early in 2008. Democrats and Republicans applied similar language to the Bush-Kerry contest. In his 1976 campaign against Jimmy Carter, Gerald Ford said, "Make no mistake – this election will decide the direction America is going to take in its third century of independence."

So every presidential race is the most momentous in modern times – until the next one.

Elections do matter, of course, because presidents make key decisions, especially about national security. Some elections matter a great deal. The 1800 fight between John Adams and Thomas Jefferson ended in the first transfer of national power from one party to the other. Sixty years later, Abraham Lincoln's victory prompted the secession of 11 Southern states, which in turn led to the Civil War. These elections shaped our national identity and profoundly influenced our political culture.

Most presidential elections aren't like that, however. The selection of candidates seldom entails a choice between radically different world futures. True, today's competitors campaign on big topics, including what will happen to social safety-net programs, such as Medicare. But the possibility that this election will pivot the country in a radically new direction is not as great as voters may think.

A president is not a monarch with unlimited power. The authority of the office is subject to the constitutional checks of federalism (sharing power with the states), bicameralism (the two houses of Congress), and the separation of powers between the executive, judicial, and legislative branches.

Presidents and their supporters feel these constraints very quickly. When Ronald Reagan won the presidency, conservatives hoped that he would shrink the federal government by scrapping the Departments of Energy and Education, among others. Facing a Democratic House, he could do no such thing, and those departments are with us 32 years later.

The limits of power also apply to presidents whose parties dominate Capitol Hill. President Obama came to office with large Democratic majorities in the House and Senate, yet he had to struggle for enactment of comprehensive health-care legislation. The version that eventually passed Congress disappointed many liberals.

Presidents must often scale back their ambitions when they learn that problems are more complex than they acknowledged on the campaign trail. In 1992, Bill Clinton pledged to welcome Haitian refugees that the Bush administration had turned away. Even before he took office, he had to backtrack when the Coast Guard warned of a massive exodus. Similarly, Obama found that he could not keep his promise to close the Guantánamo Bay prison by January 2010.

"A dependence on the people is, no doubt, the primary control on the government," wrote James Madison in The Federalist Papers. The parties may think they can win permanent majorities, but when people don't like what the president is doing, they exercise this control at the next midterm election. In 2006, they gave Republicans what President George W. Bush called a "thumping." When it was the Democrats' turn in 2010, Obama called it a "shellacking." Madison didn't use either word, but he would have understood.

Candidates often speak as if they would be immune to checks and controls, such as when Mitt Romney asserts, "I will repeal Obamacare." Presidents lack the power to pass or repeal statutes – only Congress can do that. Although chief executives can try to persuade Congress, they have no guarantee of success.

More generally, presidential contenders draw the future in caricature: If you elect me, my proposals will pass, and all will be well. If my opponent wins … well, just read the Book of Revelation.

It is understandable why they talk this way. Just imagine what an objective campaign message would sound like: "I think that my proposals are, on the whole, somewhat better than my opponent's. Of course, neither of us can be sure what the future holds, and in any case, we will have only a limited ability to affect what the government does. Either way, the country will get along, as it has for more than 200 years."

Not very exciting, is it?

It is no wonder that candidates go for apocalyptic rhetoric, and have been doing so for a long time. In 1800, a Federalist newspaper pictured what would happen if Thomas Jefferson won: "Are you prepared to see your dwellings in flames, hoary hairs bathed in blood, female chastity violated, or children writhing on the pike and halbert?"

In recent years, partisan polarization has probably added fuel to the rhetorical fire. Strong partisans often see leaders on the other side as not just mistaken, but evil. Winthrop University political scientist Scott Huffmon calls this attitude the "kick-a-puppy syndrome," meaning that people think their foes are so bad they would kick puppies.

Some Democrats say that Republicans are deliberately trying to hurt the economy so they can win the 2012 election. Conversely, Rush Limbaugh argues that Obama wants to wreck the free-market system so that the government can take over. Mr. Limbaugh has criticized Mr. Romney for conceding the president's good intentions.

Such partisans think November represents the ultimate decision on the country's fate. They ought to remember what running back Duane Thomas said of the 1971 Super Bowl when a reporter asked how it felt to play in "the ultimate game." Mr. Thomas replied: "If this is the ultimate game, how come they're playing it again next year?"

John J. Pitney Jr. is the Roy P. Crocker professor of government at Claremont McKenna College in California and coauthor of "American Government and Politics: Deliberation, Democracy, and Citizenship."