Eight years after Hillary Clinton helped unite Democrats behind Barack Obama's presidential campaign, he is returning the favour.
Mr Obama and Mrs Clinton will make their first joint appearance of the 2016 campaign in North Carolina, a state Democrats are eager to win in November.
Presumptive Republican nominee Donald Trump will hold his own event in the political battleground a few hours later.
The Democrat duo's rally in Charlotte marks a new phase in their political relationship. They were bitter rivals in the 2008 Democratic primary but became colleagues when Mrs Clinton joined Mr Obama's Cabinet as secretary of state. Now, they are co-dependants as Mrs Clinton seeks the White House once again.
Her chances of winning hinge on rallying Mr Obama's coalition to her cause. His legacy depends on her success.
Aides to both say the foe-to-friend story will be at the centre of the Obama-Clinton show on Tuesday.
In his remarks, the President will act as a character witness for his former adviser, who is struggling to convince voters of her trustworthiness and honesty.
There is no better politician to testify on her behalf, many Democrats believe, than the man who once counted himself among Clinton sceptics but came to be one of her biggest supporters.
"I think that he can be very helpful, particularly with Democratic voters and some independent voters who have doubts," said David Axelrod, the chief architect of Mr Obama's 2008 race for the Democratic nomination against Mrs Clinton.
"He can do that by sharing his own experience. They were rivals, they had their differences; that gives him some additional standing."
The Clinton campaign is also hoping that Mr Obama's presence at her side serves as a reminder of another, more popular chapter in her career.
For four years, Mr Obama trusted her to circle the globe representing his foreign policy. She sat at his side in the Situation Room. She was the good soldier, putting aside her political ego to join the administration of the man who defeated her. During her tenure at the State Department she was viewed favourably by most Americans.
"As someone who was a former rival and came to put a lot of faith in her, we believe the President's support for her is particularly meaningful to voters," said Clinton campaign adviser Jennifer Palmieri.
The White House confirmed on Monday that Mrs Clinton and Mr Obama will travel to the event together on Air Force One.
Mrs Clinton's Republican presidential rival objected to the travel plan.
"Why is President Obama allowed to use Air Force One on the campaign trail with Crooked Hillary?" Mr Trump tweeted. "Who pays?"
Presidents make all their flights on Air Force One, regardless of the purpose of the trip. Political committees are required to contribute to the cost of a president's campaign-related travel, though a portion of such costs is borne by taxpayers.
"As is the standard practice, the campaign will cover its portion of the costs," Clinton spokesman Nick Merrill said.
Mr Obama makes his first campaign appearance with Mrs Clinton during a wave of popularity unlike anything he has experienced since his first term.
Clinton aides say they are confident they could deploy him in any battleground state, though they believe he will be particularly effective in rallying young people, as well as black and Hispanic voters, and will be instrumental in voter registration efforts.
In a series of remarks in recent weeks, the President has proved himself to be one of the Democrats' most effective critics of Mr Trump. From the White House and on the world stage, Mr Obama has regularly found ways to blast Mr Trump's message and mock his style. The mix of high-minded concern and sharp-elbowed sarcasm is widely viewed as an effective, tweetable model for other Democrats.
Nevertheless, Mr Obama will not spend the next four months as the "Trump-troller in chief", as one official put it.
The President plans to take a largely positive message on the road as his campaigning picks up later this summer - partly because he is campaigning for the continuation of his agenda - as well as Mrs Clinton's. On health care, immigration, financial reform and the environment, Mrs Clinton is largely promising a continuation or acceleration of Mr Obama's policies.
Mr Obama and Mrs Clinton originally planned to make their first campaign appearance together in Wisconsin, a Democratic-leaning state where Mrs Clinton struggled in her primary fight with Bernie Sanders. Campaign aides viewed the rally as a way to forge Democratic unity after the bruising primary and consolidate the party's voters in a state she needs to carry in November.
But the June 15 rally was postponed due to the mass shooting at an Orlando nightclub. By the time the campaign and White House got around to rescheduling, Clinton aides said the landscape had shifted - they are now far less worried about attracting Sanders voters and more interested in using the President to rally voters in one of the most divided general election battlegrounds.
Mr Obama narrowly won North Carolina in the 2008 presidential election, becoming the first Democrat to win the state since 1976. His campaign aggressively registered more young people and black voters, and he drew support from moderates in the booming suburbs of Charlotte and Raleigh-Durham.
The President was eager to cement Democrats' strength in North Carolina during his re-election campaign, even holding his convention in Charlotte.
But he was dogged by a sluggish economy and disappointment among some swing voters, and lost to Republican Mitt Romney by 2 percentage points.