Scan the history of second-term presidents and you can't help but wonder why so many even bothered to run for another four years.

More often than not, second terms amount to giant busts.

"You can't find a president over the last 100 years who had a second term better than his first," said Carlton College political scientist Steve Schier.

Call it the second-term curse. Scandals, presidential overreach, a defiant Congress. All have contributed to undermining the ambitions of the nation's re-elected chief executives.

President Barack Obama faces yet another hurdle as he moves into his second four years -- the unresolved fiscal cliff, which threatens to upend his plans for his final term.

Governors aren't immune to the second-term curse either. With Democrat Jay Nixon about to begin his second four years in Jefferson City, a look at recent two-term governors in Missouri and Kansas shows they haven't done much better.

Alfred Zacher, a scholar who studied the 20 presidents who won second terms, concluded that only seven avoided "a troubled or failed second term." Of the recent presidents, Zacher rated Ronald Reagan and Bill Clinton, despite his impeachment in the wake of the Monica Lewinsky scandal, as completing a successful second four years.

Other successes: George Washington, James Madison, Andrew Jackson, Theodore Roosevelt and Dwight Eisenhower.

Based on Zacher's research, Barack Obama has about a 37 percent chance at success.

"Not very optimistic, is it?" Zacher said.

In his book, "Presidential Power in Troubled Second Terms , " ** Zacher found only one president who had a better second term than his first -- James Madison, president from 1809-1817.

"For many ... frustration, failure and even disaster followed their re-election," he wrote.

Zacher's findings are counterintuitive. By the time he reaches his second term, a president has four years experience on the job. In theory, he understands how to work the levers of power and how to deal with an always maddening Congress. But it often doesn't work that way.

"Part of it may be hubris," said University of Virginia scholar Larry Sabato. "Presidents who are re-elected may think they're more beloved than they actually are."

A second-term president's lame-duck status poses an immediate hurdle. Because they can't run again, members of Congress and governors aren't as beholden to a re-elected chief executive. Presidents only have a few months to take advantage of their second-term success before their authority wanes.

"You lose power," said Kansas State University political scientist Joe Aistrup. "You're automatically out the door."

Retaining the same top staffers who helped a president get elected in the first place is more difficult in a second term. And with time, the public grows weary of presidents who remain on the scene for so many years.

"With the public, it's a been-there-done-that attitude that grows over a second term as people try to look forward," Schier said.

The struggles of modern-era two-term presidents are well-documented. Franklin Roosevelt's greatest defeats came in his second term, including his attempt to expand the size of the Supreme Court. Harry Truman faced charges of corruption, endured a military stalemate in Korea and was compelled to oust the popular Gen. Douglas MacArthur.

Eisenhower struggled with budget fights, civil strife and a growing sense that he was indecisive. Lyndon Johnson grew increasingly unpopular as the Vietnam war lingered.

Watergate cost Richard Nixon his dreams of capitalizing on his 1972 foreign-policy triumphs in China and the Soviet Union. Reagan had to face down the Iran-contra scandal, although he managed to sign major tax-overhaul legislation and a first-of-its-kind treaty limiting nuclear weapons.

Clinton, a student of history, said after his 1996 re-election that he was well aware of the second-term curse.

"I'm very mindful of history's difficulties," he said at his first post-election news conference. "And I'm going to try to beat them."

In his second term, he advocated bite-sized proposals. Whereas in his first term, Clinton sought to pass national health-care reform, he focused on college tax credits and school uniforms in term two, although he also signed a welfare reform law.

George W. Bush started his second term with a clarion call to spread liberty and freedom "to the darkest corners of the world." But his inability to pass a key initiative, Social Security reform, hurt. So did his response to Hurricane Katrina, combined with a Democratic takeover of Congress in 2006. A final blow was an economy that cratered shortly before he left office.

Obama is up against the usual problems -- and also the ongoing fiscal cliff negotiations that stand to affect virtually every American. Failure to resolve the complex issue with its tax increases and steep budget cuts could cast a pall over Obama's entire second term and undermine his hope of achieving immigration reform and energy legislation.

"It could poison his relationship with Republicans, which means no action on anything of consequence for about two years," Schier said.

As Democrat Jay Nixon gears up for his second term in Jefferson City, the outlook isn't much rosier. Nixon begins another four years with a General Assembly firmly in Republican hands.

Republicans control 110 of 163 House seats. That's a record and is one more than the two-thirds majority needed to override a Nixon veto. In the Senate, Republicans have a 24-10 edge, also a veto-proof majority. That means that if Republicans can hang together, they can control the lawmaking process.

The last Missouri governor elected to a second term was Democrat Mel Carnahan in 1996. But the first legislative session of his second term was marked more by what didn't get done than what did. Although lawmakers agreed to Carnahan's 3-cent cut in the food sales tax, two other major priorities -- a children's health-insurance program and college-tuition tax credits -- were defeated. He also found himself embroiled in caustic clashes with members of his own party.

The children's health-insurance program passed the next year, but his final two sessions were largely lackluster.

"Traditionally, governors don't have the same pull in their second term," state Sen. Ronnie DePasco, a Kansas City Democrat, said at the time.

In Kansas, the last governor who completed a full second term was Republican Bill Graves, who left office in 2003. His final two years were marked with dramatic policy reversals. Instead of cutting taxes as he had during his early years in office, a national economic slowdown forced Graves to raise them -- and order budget cuts totaling tens of millions of dollars.

He also faced criticism that he hadn't taken advantage of his personal popularity to do more for public education.

"Had Bill Graves left office in 2000, he would have been regarded as, if not one of the best, certainly a first-line governor," University of Kansas political scientist Burdett Loomis said at the time. "In the end, almost all governors face some truly tough times. It's not clear he passed that test very well."