Who Pays When the President Travels for Campaign Events?

— -- Presumptive GOP nominee Donald Trump took to Twitter Tuesday to express outrage over the cost to taxpayers for President Obama’s campaign trip to Charlotte, North Carolina with Hillary Clinton.

But what is the actual cost to taxpayers?

In the case of political travel by the president, there are rules that the federal government be reimbursed by the appropriate political organization for the costs of the travel.

White House Press Secretary Josh Earnest told reporters aboard Air Force One that today’s trip, for the president’s first joint campaign appearance with Hillary Clinton as the presumptive Democratic nominee, was no exception to that rule.

“The White House of course follows all rules and regulations that apply to the president’s political travel,” Earnest said, explaining that the DNC discloses payments made to the federal government for such purposes on a monthly basis to the FEC in compliance with the regulation.

In the most recently available figures, the cost of operating Air Force One averages approximately S180,118 per hour, according to the Air Force. This figure includes fuel, food, repairs, and basic maintenance.

"As is the standard practice, the campaign will cover its portion of the costs," a Clinton aide told ABC News.

But while relevant political organizations may be on the hook for paying part of the costs associated with the plane’s operation, there are additional security costs associated with presidential travel that do fall to the taxpayers.

Richard Painter, a former chief White House ethics lawyer under President George W. Bush, said there’s a well-established process for sorting which costs can be charged to political organizations.

“There’s a process that is used to allocate the costs between the DNC or the relevant campaign and the government,” Painter told ABC News. “The cost of the Secret Service is paid for the taxpayer. If the president goes on vacation, we run into the same thing.”

It’s an unavoidable problem, Painter said, for presidents who are simultaneously commanders-in-chiefs and the leaders of their respective political parties.

“I don’t think this is controversial,” Painter said. “A president that won’t campaign for his own party isn’t the leader of his own party. If someone claimed that President Bush was abusing taxpayer money to campaign, we would have laughed at that.”

In the case of political figures flying aboard Air Force One, as was the case today with Hillary Clinton and several of her campaign aides who also traveled with her, Painter said they did not get a free ride.

There’s a formula by which they are charged for their flight comparable to the cost of a commercial first class ticket, he said.

Trump was accompanied by Secret Service agents when he went on a recent business trip to Scotland.