WASHINGTON » History says President Barack Obama should brace for another round of midterm election losses next year — and be grateful for the opportunity.

Unlike the one-term presidents who never got the same chance, Obama, who is only the fifth president since World War II to be re-elected, at least has the possibility that his party might hold or gain ground in Congress in his sixth year in office.

But the unhappy record of his two-term predecessors — none of whom gained control of either legislative chamber — offers scant comfort about his prospects.

Dwight D. Eisenhower in 1958 set the standard for sixth-year losses. With the nation reeling from economic recession and the Soviet Union’s launch of the Sputnik satellite, his fellow Republicans lost 13 seats in the Senate and 48 in the House, turning narrow Democratic majorities into overwhelming ones.

After Eisenhower, five straight presidents did not serve two full terms — John F. Kennedy, Lyndon B. Johnson, Richard M. Nixon, Gerald R. Ford and Jimmy Carter — but then came Ronald Reagan, who approached Election Day 1986, his sixth year in office, in a stronger position. The economy was growing, and he had signed an overhaul of the tax code that slashed the top rate to 28 percent.

Still, that only partly shielded his party from voters’ wrath. Although the House Republican minority lost just five seats in the midterm elections, well below the historical average for the president’s party, the Republicans lost eight Senate seats and handed the chamber over to Democratic control.

President Bill Clinton, who was under siege over the Monica Lewinsky affair when voters went to the polls in 1998, had help from an economic boom and Republican adversaries who overplayed their hand. But even though he became the first White House incumbent since Franklin D. Roosevelt to see his party gain House seats in any midterm contest, that unlikely pickup was not enough to put Democrats back in control.

In 2006, President George W. Bush, like Eisenhower, suffered a defeat so unequivocal that he called it "a thumping." Hobbled by the unpopular Iraq war and criticism of the administration’s handling of Hurricane Katrina, Republicans lost control of both the House and Senate for the first time in 14 years.

Presidential approval ratings do affect sixth-year midterm prospects, although they predict only so much.

Bush’s sub-40 percent rating made him a fat target for 2006 Democrats, and today, Obama’s decline to an approval rating in the low 40s after the disastrous rollout of his health care law has lifted Republican hopes.

Yet Reagan’s approval rating topped 60 percent before he lost the Senate in 1986 as several weak Republican incumbents he had helped sweep into office six years earlier fell to Democratic challengers.

"I don’t think there are any formulas" for midterm election results, said Ken Khachigian, a former Reagan speechwriter. "We underplay the fact that elections are elections with individual candidates."

The state of voters’ pocketbooks also plays a role. In 1958, weak growth of 1.1 percent in Americans’ disposable incomes hobbled Eisenhower’s Republicans, while the robust growth 40 years later of 5.9 percent helped Clinton and his party.

Using historical patterns, Gary Jacobson, a congressional election expert at the University of California, San Diego, calculates that U.S. incomes in 2014 would need to grow 3.8 percent above current forecasts for Obama’s party to break even in next year’s House elections. Yet 4 percent growth in 2006 could not protect Bush’s fellow Republicans.

One long-term trend that has shrunk both midterm risks and rewards for the president’s party and its opposition is polarization, which has sharply diminished the number of competitive House seats.

In 1986, 45 percent of all House seats were held by members whose districts had voted for the other party in the previous presidential election. That proportion fell to 26 percent by 1998 and 14 percent by 2006.

Today it stands at 6 percent, or 26 seats. That "huge structural change," Jacobson said, limits potential swings in either direction.

Divided control of government gives 2014 Democrats additional insulation that Bush and his party lacked in 2006. But Obama’s reliance on young and minority voters — those most likely to remain indifferent to midterm contests — makes his party vulnerable to an unusually large drop-off in turnout.

In addition, both sides are vulnerable to rapid shifts in the election-year agenda.

Just weeks ago, Democrats savored the idea that an unpopular government shutdown might help them scrape together the 17-seat gain they need to win the House back. The polls had given them hope: In early October, an NBC News-Wall Street Journal poll showed that Americans, by an 8-percentage-point margin, wanted Democrats to control Congress. The two parties had been tied on that issue in July.

Within three weeks, as attention shifted to problems with the new health care law, that lead had fallen by half, raising Republican hopes of gaining the six seats they need for a Senate majority.

Roughly 48 more weeks remain before Election Day. "Republicans are getting ahead of themselves to think Obamacare is going to be the definitive issue," Khachigian said.

In fact, at this point in the 1998 election cycle, in December 1997, the Lewinsky scandal was still six weeks from the headlines.

David Plouffe, then a top House Democratic aide and more recently senior adviser to Obama, recalled that by the spring of 1998, "most observers said Democrats were cooked."

"By the fall, things were different," Plouffe added. "Be careful of predictions."