WASHINGTON - Second presidential terms are never easy - even for George Washington.

More often, they're fraught with peril, frequently marred by scandal, failure, hubris, and burnout, and souring relations with Congress.

President Barack Obama acknowledges the dangers of overreach but vows to steer cautiously. The odds are against him.

He's the 20th U.S. president to serve all or parts of two terms. Most of the others have encountered setbacks and frustrations.

He's also the third in a row to win a second four-year term. Both predecessors stumbled.

President Bill Clinton was impeached by the House over lying about an affair with White House intern Monica Lewinsky, although the Senate declined to remove him from office. President George W. Bush failed to get a big Social Security overhaul through Congress and was slammed for his handling of Hurricane Katrina and growing voter anxiety over the Iraq and Afghanistan wars.

From Inauguration Day, a second term president's influence and power begin to ebb.

"It's called fatigue, people burn out. Typically, the top people are recruited for the first term. For the second term, you kind of go to the bench," said Ross Baker, a political science professor at Rutgers University. "It's a little less illustrious than the starting lineup. You're going to get more people perhaps a little less sure-footed. That's putting it, perhaps, mildly."

There's something of a political Continental Divide with second terms. At some point everybody's attention starts flowing in the other direction as those in both parties start shifting their focus to the next election.

Also, Obama sets out against a backdrop of looming new fiscal showdowns that will come to a head in March - another battle over the debt limit, mandatory spending cuts postponed from January and the expiration of spending authority for the entire government.

And some of his top second-term goals such as immigration and tax-code overhaul, gun control and climate-change legislation come as grim budget realities cast a long shadow over what he can accomplish.

History is littered with troubled second terms.

Richard Nixon resigned in disgrace. Ronald Reagan's second term was marred by the Iran-Contra guns-for-hostages scandal.

Even George Washington, the nation's revered first president, had an ugly second term.

His backing of the Jay Treaty expanding trade ties with Revolutionary War foe Britain divided the nation. Many leaders - including future president Thomas Jefferson - challenged Washington. Jefferson called the treaty a "monument of folly." Angry crowds gathered outside Washington's house and talk simmered of impeachment.

Franklin D. Roosevelt was able to win four terms but had a tumultuous second one despite a 1936 re-election landslide. His effort to expand and pack the Supreme Court with ideological allies was soundly rebuffed by Congress. And Democrats suffered mightily in the 1938 midterms.

Since Roosevelt's day, presidents have been constitutionally limited to two terms. The downside for victorious incumbents: being unable to run again limits a second-term president's clout, lessening the ability to reward allies or thwart political foes and hastening lame-duck status.

But second terms don't have to be failures - and Obama won't necessarily succumb.

William Galston, a domestic policy adviser in the second Clinton administration, said the notion of a second-term jinx or curse is an over-simplification because "a lot of presidents have trouble in their first terms" and don't get re-elected. And second-term achievements - such as Clinton's - need to be weighed along with setbacks, he said.

Galston also suggested some things may be easier for Obama in his second term given the dynamics of his re-election victory - such as immigration and tax-code overhaul. He's already gotten Congress - post-election - to raise taxes on the wealthiest Americans, something he couldn't do earlier.

Clinton's second term? "I would judge it as an incomplete success. And its incompleteness is largely his own fault," said Galston, a senior fellow at the Brookings Institution.

In his book, "Presidential Power in Troubled Second Terms," presidential scholar Alfred Zacher concluded that only one president had a truly better second term than his first: James Madison, president from 1809-1817.

In Madison's first term, the still-new nation was drawn back into armed conflict with Britain in the War of 1812. But in Madison's second term , the U.S. scored a dramatic victory in the Battle of New Orleans and the war ended with the Treaty of Ghent. Madison's popularity surged.

Seven others had OK second terms despite setbacks, Zacher wrote, most recently Dwight Eisenhower, Reagan and Clinton.

Reagan, despite Iran-Contra, oversaw a major 1986 simplification of the tax code and the unraveling of the Soviet Union in his second term. Clinton learned how to reach across the aisle to deal with Republicans on welfare overhaul and deficit reduction and left office with an annual budget surplus - an achievement no other president since Andrew Jackson can claim.

James Thurber, director of the Center for Congressional and Presidential Studies at American University, said that by the end of a first term, "The American people have gotten to know the president very well. The enthusiasm of his first election is long gone. That limits the possibility of great success in the second term."

Also, presidents who were just re-elected can get overconfident from their victories, see their elections as a "mandate" to push their agendas, start believing re-election campaign hype and surround themselves with many burned-out senior staffers and inexperienced younger ones, suggests Kenneth M. Duberstein, who was Reagan's chief of staff.

"Add to this too much communal drinking from the same Kool-Aid, and it is a recipe for disaster for any second-term president," Duberstein wrote. "All of us who have served in second-term White Houses have seen this witch's brew."

President George W. Bush claimed "political capital" from his 2004 re-election win and "I intend to spend it." But it was a losing investment. He got little done in his second term.

Thomas Cronin, a political science professor at Colorado College, said Obama's time to wield influence is limited and he needs to strike quickly.

A year or two into a president's second term, "people say, well, let's just wait this guy out," Cronin said." That's why the next six to 10 months are so crucial."