Last week was a tough one for Barack Obama.

The president’s choice for HHS secretary withdrew on Tuesday. It turned out that Tom Daschle, who considered himself up to the task of redesigning the most complex and fastest-growing sector of our economy, had trouble figuring out his own taxes.

By the end of the week, Obama was facing growing resistance to key parts of his $800-plus billion stimulus package. Friday found the new president recuperating at Camp David.

Welcome to the NFL, Barack: There will be many more tough weeks to come.

The “Hopefest 2009” aura that surrounded Obama’s inauguration made him appear unstoppable. But the smart money says that by 2012, Obama will look a lot more like Jimmy Carter than FDR. That’s not because the new president is incompetent; it’s because he’s signed up for an impossible job.

Our Constitution’s framers had a modest view of presidential responsibility: the president was, in Washington’s phrase, the mere “chief magistrate,” and his main job was faithful execution of the laws.

But today, Americans look to the president as the Savior-in-Chief, a figure who will heal what ails us—whether it’s unemployment, hurricanes, divisiveness, or spiritual malaise. When it comes to the presidency, we demand what we cannot have and, as a result, we usually get what we do not like.

Political scientists have a term for the vast distance between what the public expects of the president and what he can realistically deliver: the “expectations gap.” And no presidential candidate in living memory has done as much as Obama to stoke public expectations for the office—which were insanely high to begin with.

“Yes we can!” was the preferred hosanna of hope in the revival-tent atmosphere of the Obama campaign. We can, Obama promised, create a “new kind of politics,” “end the age of oil in our time,” deliver “a complete transformation of the economy,” and even “create a kingdom right here on earth.” With the presidency, it seems, all things are possible.

Post-election polls suggested that Americans bought the sales pitch. Eight in 10 expected Obama to improve conditions for the poor, 70 percent to improve education and the environment, and 60 percent counted on him to create a robust economy.

Obama entered office with a 79 percent favorability rating, the highest score of any newly elected president since, well, Jimmy Carter.

As the Carter experience suggests, in presidential politics, great expectations often lead to crashing disappointments. Every post-WWII president has faced what scholar Barbara Hinckley called “the decay curve”—the decline in popularity that occurs as the public recognizes that the president can’t deliver the miracles he’s promised.

String them together, and presidential approval graphs look like an EKG on a patient being repeatedly shocked to life—“clear!”—and then fading out again. Just as popularity tends to fade within each president’s tenure, average approval ratings have been in decline from one president to the next for most of the modern era.

You’d never know it from his budget-busting economic nostrums, but Obama has taken office in an era of limits. And when he fails to fully heal our financial troubles, fix health care, teach our children well, provide balm for our itchy souls, and so forth, his hope-addled rhetoric will seem all the more grating, and the public will increasingly come to see him as the source of all American woes.

Perhaps, then, we ought to drop the notion of president as Savior-in-Chief. Our Constitution’s Framers thought the president had an important job, but they never looked to him to heal all the nation’s wounds and save the national soul.

Their vision of the presidency may be unromantic, but at least it’s realistic (not to mention cheaper). Until we return to the framers’ modest, businesslike view of the presidency, we shouldn’t expect any president, however well-intentioned, to be “a uniter, not a divider” in American life.