David Jackson

USA TODAY

WASHINGTON — An already-unusual election is developing another unique feature: An outgoing president who is busy on the campaign trail.

Little more than seven months before the end of his administration, President Obama is poised to become the most active lame duck campaigner in history, offering a new twist on an often awkward role: A White House occupant watching the election of a successor.

"They usually wait to pretty much close to the end, when it really starts to heat up," said Stephen Hess, a former aide to presidents Dwight Eisenhower and Richard Nixon. "This man is in it very early, which means he will be in it very long."

Obama has served notice he is willing to work hard to elect a Democratic successor — most likely Hillary Clinton — and defend his own legacy, currently under assault by Republican nominee-in-waiting Donald Trump.

Trump's attacks may well inspire toward "a record-breaking amount of intensity, energy, and time invested on the campaign trail." said historian Gil Troy, whose many books include See How They Ran: The Changing Role of The Presidential Candidate.

In the past, lame duck presidents have been inhibited from campaigning too much, either because of low approval ratings or friction with their party's nominees.

Obama has already made his presence felt on the 2016 campaign, frequently criticizing Trump as temperamentally unfit for the presidency. In speeches last week in Indiana and at the Air Force Academy in Colorado, the president made mocking references to a variety of Trump's domestic and foreign policies.

"If the economy is really what’s driving this election, then it's going to be voters like you that have to decide between two very different visions of what's going to help strengthen our middle class," Obama told a supportive crowd in Elkhart, Ind.

Trump, meanwhile, says that if Obama campaigns again, he is only too happy to return the favor, telling backers in Redding, Calif.,"Once they attack, then we're allowed."

Throughout history, presidents have taken an active interest in the elections of their successors. After all, these races have often served as comments on their own terms in office — an "insta-verdict," said Troy, a history professor at McGill University in Montreal.

Ahead of the contest in 1836, President Andrew Jackson worked hard behind the scenes for the election of his vice president, Martin Van Buren. In 1908, President Theodore Roosevelt pushed for the election of his hand-picked successor, William Howard Taft.

For more than a century, it was considered bad form for presidents to actively campaign, even in their own elections. Lame duck presidents have been more active on the stump in recent decades, to the degree they are welcome.

Presidents with low approval ratings — like George W. Bush in 2008 and Lyndon Johnson in 1968 — were liabilities for the party candidates seeking to succeed them (John McCain and Hubert Humphrey).

Personal dynamics can also create problems for presidential candidates. President Dwight Eisenhower badly undercut Vice President Richard Nixon's campaign in 1960 with an infamous comment during a news reporter. Asked to name one idea Nixon had contributed to his administration, Eisenhower responded: "If you give me a week, I might think of one. I don't remember."

During the 2000 election, Vice President Al Gore discouraged President Bill Clinton from campaigning for him, less than two years after the Monica Lewinsky and impeachment episodes. (Clinton, of course, has been busy campaigning for wife Hillary in the current contest, another unusual aspect of Campaign '16).

The closet comparison to Obama may be Ronald Reagan, a popular incumbent who headlined at least 20 party rallies ahead of Vice President George H.W. Bush's election in 1988.

Obama doesn't appear to have historic handicaps as he hits the campaign trail in his lame duck year. His approval ratings have inched above 50%, and both Clinton and Bernie Sanders have made clear they would welcome the president's support.

With the Democratic primary season winding down and Hillary Clinton almost certainly in a position to claim enough convention delegates to clinch the nomination Tuesday, Obama is expected to issue a formal endorsement soon. Joint appearances with Democratic candidates up and down the ballot are expected throughout the year.

Another unique factor this election: Trump. His feud with Obama goes back to the time when the New York businessman challenged whether Obama had been born in the United States. The president responded by producing a long-form birth certificate showing he was born in Honolulu, Hawaii.

Historian Matthew Dallek, an assistant professor of political management at George Washington University, said Trump is a unifying force for the Democrats, and an inspiration for an aggressive campaign from Obama.

"It is something of a departure from what we've seen in previous elections," Dallek said. "But it's not surprising."

Hess, a senior fellow emeritus at The Brookings Institution, noted that Trump has vowed to roll back Obama's policies, meaning the president's legacy is very much at stake in this election.

"He's not only in this for Clinton," Hess said. "He's in this for himself."