Barack Obama is returning to the political arena for the first time in months after keeping a low profile and avoiding direct confrontation with his White House successor.
The 56-year-old former president is scheduled to attend campaign rallies in New Jersey and Virginia on Thursday to support Democratic party candidates for governor.
Voters in both states will decide the gubernatorial contests on November 7, one year after Donald Trump defeated Democrat Hillary Clinton and stormed into the White House on a wave of anti-establishment fury.
The races are a potential indicator of voter sentiment ahead of the 2018 midterm elections, which will be a major test for Trump and his Republican party.
Political science professor Larry Sabato said the New Jersey and Virginia governor races are the only "big elections" for 2017.
"What's at stake is bragging rights headed into the 2018 midterm elections," Sabato told AFP.
It is unclear what Obama's message will be. The former US leader has remained largely detached from the political debate since leaving office on January 20, in keeping with presidential tradition.
Trump has meanwhile used his first nine months in the White House to methodically demolish key Obama administration policies.
After three months of vacation Obama began writing his memoirs. He has said little in public and granted almost no interviews.
The few times Obama broke his silence was to comment on issues of national importance, such as immigration, health care and climate change.
But the 44th president may be tempted on Thursday to take aim at Trump, who has frequently and publicly excoriated his predecessor.
- TEST FOR TRUMP -
In New Jersey, the post of governor will almost certainly go to Democrat Philip Murphy, who would succeed Chris Christie, a Trump ally whose popularity has plummeted to record lows.
New Jersey "is a runaway win for the Democrats, so Virginia is the only competitive contest. Obama is needed much more in Richmond than Trenton," said Sabato, referring to the capitals of the two states.
Virginia is a pivotal state and the only southern US state that Clinton won in 2016. Its importance is amplified by its proximity to the US capital.
"If the GOP loses in Virginia, Trump will be widely blamed since he is so unpopular in a state carried by Hillary Clinton," Sabato said.
"Should the Republicans win Virginia's governorship, then Trump will not be viewed as such a liability for the GOP in 2018."
In Richmond, Obama will back Ralph Northam, a former military doctor who was credited Wednesday with a slight lead over Republican Ed Gillespie in a Quinnipiac poll.
Obama's impending arrival in the city of over 220,000 people sparked long lines of people seeking tickets.
More than six hours ahead of the event, Washington student Lucas Anderton was in the queue.
"It's important for me, he's my hero and so it's nice to see him out in the battle again," Anderton said.
"The Hillary Clinton era lost a lot of the Democratic base that Obama had built up, so hopefully having Obama campaigning for him will help build that base back up."
Well aware of the vote's importance, Trump has backed Gillespie and accused Northam of "fighting for the violent MS-13" Hispanic gang, as well as "sanctuary cities" that offer shelter to illegal immigrants.
Gillespie, a former advisor to president George W. Bush who has become a millionaire lobbyist, has so far kept a cautious distance from the mercurial Trump, whose backing recently failed to ensure the election of his pick in a Republican Senate race in Alabama.
