Washington (CNN) President Barack Obama will give Hillary Clinton a lift on Air Force One Tuesday when they fly to North Carolina for their first joint campaign appearance.

It's a powerful symbol of the presidency that Obama appears happy to confer upon his preferred successor. Clinton's presumptive Republican opponent repeatedly criticized the move Monday and Tuesday.

"Taxpayers are paying a fortune for the use of Air Force One on the campaign trail by President Obama and Crooked Hillary. A total disgrace!" Trump tweeted

Taxpayers are paying a fortune for the use of Air Force One on the campaign trail by President Obama and Crooked Hillary. A total disgrace!

On Monday afternoon, he also slammed the trip.

"Why is President Obama allowed to use Air Force One on the campaign trail with Crooked Hillary?" Donald Trump tweeted Monday. "She is flying with him tomorrow. Who pays?"

Why is President Obama allowed to use Air Force One on the campaign trail with Crooked Hillary? She is flying with him tomorrow. Who pays? — Donald J. Trump (@realDonaldTrump) July 4, 2016

It's a question presidential candidates have asked of their incumbent rivals for years, and the answer is complicated.

The cost of flying Air Force One for political travel is divided between the federal government (using taxpayer dollars) and the candidate's political organization. Because Obama is traveling to Charlotte solely for the purpose of stumping for Clinton, her campaign (or the Democratic National Committee) would have to foot a portion of the bill whether the candidate was aboard or not.

The exact breakdown of how much the campaign will owe, however, isn't precisely known. In 2012, when Obama was running for re-election, his campaign reimbursed the government millions of dollars for Air Force One travel based on a pro-rated share of an equivalent-sized charter plane.

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Travelers who must reimburse the government for political trips include the candidate and any staff traveling on behalf of her campaign. Other passengers, including security personnel and medical staff, aren't required to reimburse the government for their portion of the trip.

Whatever Clinton's campaign does pay, however, is far less than the actual costs of flying Air Force One, which is retrofitted with secure communication and navigation equipment, and costs north of $200,000 to operate per hour.

Tuesday won't be the first time Obama has given Hillary Clinton a lift on Air Force One. In 2013, the pair flew to Nelson Mandela's funeral in South Africa together (along with former President George W. Bush and his wife Laura).

Before that, Clinton flew with Obama occasionally as Secretary of State, including to Myanmar in 2012.

While former Presidents Bill Clinton and George W. Bush largely stayed off the campaign trail in their final years in office, Obama is expected to stump heavily in support of the presumptive Democratic nominee, meaning Democrats will be on the hook for at least some of his travel in the coming months.

Like past administrations, Obama could offset some of the price-tag by combining political travel with official events, which are paid for by the government. The formula breaking down campaign and official costs, however, has been kept secret by White Houses going back to the 1970s.

Obama also hasn't shied away from ripping Trump at "official" events over the past year, including most recently during a foreign leaders' summit in Canada.