More than at any time in memory, the United States faces a choice this fall between unpopular presidential candidates. Most voters view Hillary Clinton and Donald Trump with disdain.

Yet one popular politician looms over the contest. That’s President Barack Obama, who this spring has seen his approval rating again creep past 50 percent. For Democrats hoping to hold the White House, that has come at just the right time.

Obama’s standing marks a final-year turn in the arc of a presidency defined largely by the intensity of his opposition. But it also colors the November battle to choose his successor.

Notwithstanding hopes to the contrary when his career began, the nation’s first African-American president has proved powerless to reverse a decades-long trend toward political polarization. As a result, his average approval rating in the Gallup Poll has rarely surpassed 50 percent for six of his seven full years in office.

His yearlong average fell from 57.2 percent in 2009 to 46.7 percent in the midterm election year of 2010, when Republicans regained control of the House of Representatives. It dropped to 42.6 percent in 2014, when Republicans won the Senate, too.

Obama has rebounded sharply from there. This year, his first-quarter Gallup average was 49.5 percent, his highest since just after his second inauguration in 2013. His average for May has been just over 51 percent.

History suggests that a president’s approval rating can help predict who replaces him. The opposing party has captured the White House in every race since World War II in which the current president’s approval rating was below 45 percent in late spring of their final year: Harry S. Truman in 1952, Lyndon B. Johnson in 1968 and George W. Bush in 2008.

Those with stronger ratings fared somewhat, if not uniformly, better. Six in 10 Americans approved of President Dwight D. Eisenhower, a Republican, in 1960. John F. Kennedy, a Democrat, won narrowly anyway.

After the late 1990s economic boom, President Bill Clinton commanded robust majority support despite an infidelity scandal that culminated in his impeachment. His vice president, Al Gore, won the national popular vote but lost the race in the Electoral College.

The only departing postwar president whose party captured a third consecutive White House term was Ronald Reagan. In 1988, when Reagan recorded a yearly Gallup average of 53 percent, his vice president, George Bush, won comfortably.

Several factors contribute to Obama’s uptick. The economy has kept growing, driving down the unemployment rate and federal deficit. He reached an agreement with congressional Republicans to end the budget and debt-ceiling wars that began in 2011.

Unable to get much from Capitol Hill, he has all but stopped trying. That largely removes him from partisan combat amid a raucous, often circuslike campaign that underscores his steadiness and calm.

That above-the-fray dividend represents a perishable asset. Inserting himself actively into the campaign, as aides say Obama plans to do, will re-energize his opposition. Given a successor’s ability to strengthen or weaken his legacy, any president would consider that price worth paying.

His party still faces headwinds. In a New York Times/CBS News poll in May, 63 percent of Americans described the nation as heading in the wrong direction, extending a pessimistic mood lasting more than a decade. Just 36 percent said Clinton could bring “real change” to Washington, while 51 percent said Trump could.

Where Obama may be able to help most is in rousing elements of the coalition that twice elected him. In the Democratic race against Sen. Bernie Sanders of Vermont, Clinton has struggled to appeal to younger and independent voters. Democratic leaders have grown anxious about when, and how enthusiastically, Sanders will rally behind Clinton should she amass enough delegates to clinch the nomination after the final primaries next week.

But she holds an insurance policy with the man who defeated her in the 2008 primaries and won the presidency. Among voters ages 18 to 29, Obama drew 63 percent job approval in the Times/CBS News poll. Among Sanders supporters overall, his approval stood at 83 percent.

© 2016 The New York Times Company