WASHINGTON -- As a member of Congress for years, Leon Panetta often heard complaints about gasoline prices. He'd look up Pennsylvania Avenue toward the White House and think that the president should do something about it.

All that power to be applied -- domestically, diplomatically. "Surely the president has the ability to do something," he thought.

Then Mr. Panetta went to the White House himself, first as the director of the Office of Management and Budget, then as the chief of staff to President Bill Clinton. He found that there wasn't much a president could do to bring down the cost of gasoline. The office wasn't that powerful.

In the heady days of a presidential campaign such as this one, when candidates John McCain and Barack Obama are wrapped in the majesty of traffic-clearing motorcades and surrounded by adoring fans, when they promise to deliver great things within days of taking office, Americans should remember that the reality of governing can be far, far different.

Cut through the spin -- think of how many times you've heard a president called the most powerful man in the world or been told they "run" the country -- and the fact is that the president isn't omnipotent. Not even close.

"While we have an image of them as superheroes, they are much weaker and more limited than we give them credit for," said Michael A. Genovese, a political scientist at Loyola Marymount University in Los Angeles and a scholar of presidential power.

"You certainly could be blinded to the facts if you listen to campaigns," Mr. Panetta said. "They're all about what new candidates can get accomplished. The reality is there are real limits. ... Every presidential election is a period of great hope followed by crushing disappointment."

There are exceptions, of course, when presidents of great political talent such as Franklin Roosevelt, Lyndon Johnson or Ronald Reagan match their skills with moments of national hunger for change and accomplish more than most.

More often, however, presidents find themselves hamstrung by a system that the Founding Fathers, weary of overreaching monarchs, designed to check, not magnify, presidential power.

Just look at the Bush presidency.

Despite the aura of vast power surrounding the way that President Bush led the country to war in Iraq or acted unilaterally to combat terrorism, recent headlines underscore his limits rather than his reach.

Recently, for example, the Supreme Court ordered that he must allow terrorism suspects held at the Guantanamo Bay naval base in Cuba to appeal their cases to a U.S. court.

Last week, Mr. Bush urged that oil drilling be allowed off the U.S. coast. He can lift an executive order banning such drilling that was first signed by his father, but he also must convince Congress to lift its moratorium, which is highly unlikely.

"It is a fairly powerful office," White House spokesman Tony Fratto said. "But we have three equal and distinct branches of government. And just last week we saw one of those branches exert its muscle over the other two."

Pressed as to why Mr. Bush couldn't revoke the executive order banning offshore oil drilling, Mr. Fratto added, "that would do nothing until Congress lifts its moratorium."

Even when a president's party controls Congress -- as Mr. Bush's did for most of his first six years in office, and Mr. Clinton did his first two years -- he can find it difficult to get his way.

Mr. Clinton couldn't get an economic stimulus package through Congress and never even got a vote on his proposed expansion of health care, two of the key proposals of his 1992 campaign.

Mr. Bush pushed through temporary tax cuts in his first term, but couldn't get Congress to make them permanent. He also couldn't get his way on overhauling Social Security and immigration policy or opening the Arctic National Wildlife Refuge to oil drilling.

Even as he and Congress agreed on a tax rebate plan this year to stimulate the faltering economy, Mr. Bush faces the reality that a president doesn't have nearly as much power over a $14 trillion economy as he'd like.

"We have the image of them making momentous decisions," Dr. Genovese said. "But the story we tell is different than the day-to-day reality. That's especially true of economic policy, where the market has much more power and the Federal Reserve has more authority."

Mr. McCain or Mr. Obama could face similar challenges.

Mr. McCain wants Congress to make the Bush tax reductions permanent, a tough sell at best in Congress, which is likely to remain under Democratic control. Mr. Obama wants to expand health care, which could be difficult if the Democrats remain short of the 60 votes that are needed to get anything controversial through the Senate. If presidents can't do such things on their own, and can't count on Congress to go along, how can they do anything?

One way is to have broad support, particularly in a time of crisis such as the 2001 terrorist attacks. Mr. Bush's approval rating soared past 90 percent, and for a time he was able to get anything through Congress, including an authorization to invade Iraq and the USA Patriot Act, which gave the federal government new powers to combat terrorism at home. It's doubtful that either could pass Congress today.

Eventually, most presidents grow frustrated with the limits on them at home and turn their attention overseas.

"You can do things militarily. You can affect trade. You can establish relations with a country. Presidents generally find out that, when it comes to their foreign policy role, they have much more control," Mr. Panetta said.

Yet as Mr. Bush has discovered, even that power has its limits.

In Afghanistan, Mr. Bush was able to lead a successful invasion, but he hasn't been able to put down Taliban resistance. In Iraq, he's found that even hard-won military success hasn't brought promised political reconciliation or burgeoning Mideast democracy.

"He thought the world could be bent to his desired shape," Dr. Genovese said. "But reality has a way of kicking you in the behind."