President Trump boards Air Force One as he departs for Pennsylvania on Thursday. (Kevin Lamarque/Reuters)

President Trump has reveled in smashing political precedent, but on Thursday he followed his White House predecessors in one of Washington’s usually unspoken traditions: blurring the lines between a campaign trip and official business.

Jetting aboard Air Force One for a quick day trip to western Pennsylvania, Trump delivered a 25-minute speech at a heavy equipment manufacturing company. Though aides touted the visit to the facility in Coraopolis as a chance for Trump to promote his legislative agenda, an ulterior motive was not so thoroughly disguised.

“Will be going to Pennsylvania today to give my total support to RICK SACCONE,” Trump wrote in a morning tweet. He was referring to the Republican candidate in a closely contested congressional special election in the district where the president was headed. “Great guy,” Trump declared.

Reporters quickly noted the discrepancy, prompting Trump’s press secretary to issue a statement reiterating that the trip was, in fact, official business. But by then, Trump had given voice to a reality of the modern presidency — mixing politics and policy comes with the job.

“It’s an evergreen issue that every White House is criticized for and every White House still pursues,” said Jen Psaki, who served as White House communications director under President Barack Obama.

An attendee waves a T-shirt as President Trump speaks following a tour of the H&K Equipment Company in Coraopolis, Pa., on Thursday. (Mandel Ngan/AFP/Getty Images)

To Psaki, the reason is benign: The most valuable tool a president has, she said, is his time. Though the president is the leader of the nation, each is also the head of his political party and must manage both duties accordingly. That means advancing their policies, but also raising money for their campaigns, visiting swing states and stumping for candidates.

Presidents stretching back to at least Ronald Reagan have unabashedly doubled up on their to-do list while planning presidential travel, especially in election years, giving policy speeches by day and attending fundraisers at night.

Government watchdogs have raised red flags over such dual-purpose jaunts and who pays for them. By law, the political parties must pay a prorated share of the costs, under an opaque formula established in the Reagan era, while taxpayers are on the hook for the rest.

The problem, experts said, is that it is virtually impossible for the public to know exactly how the formula is applied.

“Where the gray area is in the formula is that we do not know what Air Force One costs,” said Lawrence Noble, general counsel at the Campaign Legal Center. “They’re not required to charge the cost of Secret Service, but they are for everybody else who travels for campaign reasons. That’s hard to decipher.”

The latest estimates from the military peg the costs of operating the presidential plane at about $206,000 an hour. When the rules were first drafted, parties were required to reimburse the government at a rate of a first-class ticket on a commercial plane.

In 2009, the Federal Election Commission amended the formula to comply with an ethics law approved by Congress two years earlier.

President Trump greets Republican House candidate Rick Saccone after speaking at H&K Equipment Co. on Thursday in Coraopolis, Pa. (Keith Srakocic/AP)

The new rules require reimbursement equivalent to the costs of a chartered Boeing 737 jet — even though the president traditionally flies on a larger Boeing 747.

The changes foisted more of the costs onto the parties. In 2004, the Republican Party reimbursed the government $1.3 million for “airlift operations,” but in 2012, the Democratic Party had surpassed that total with six months remaining before Election Day.

Yet calls of malfeasance from critics can come across as political opportunism.

In April 2012, the Republican National Committee filed a formal complaint with the Government Accountability Office over Obama’s trips to battleground states requesting an inquiry into what then-RNC Chairman Reince Priebus called “a misuse of government funds.” (Priebus served as Trump’s White House chief of staff for his first six months in office.)

On Thursday, the script had flipped. “By law the campaign or a political committee now owes the government the cost of the trip,” Dan Pfeiffer, a former senior White House adviser to Obama, wrote on Twitter in reference to Trump’s Pennsylvania visit.

Noble, the ethics lawyer, said Trump’s tweet resided in a gray area of the law.

“Is the tweet enough?” Noble said, speaking in an interview before Trump’s remarks. “If he gives a policy speech, the tweet is not enough. If he starts talking about the candidate, with the tweet, it could be.”

As Trump disembarked in Pittsburgh, Saccone was among the group of local dignitaries to greet him on the tarmac. And although the president mentioned him only briefly from the podium, Trump told reporters during a tour of the manufacturing plant that Saccone was a “special person” and promised to return for a full-fledged campaign event.

Ari Fleischer, a press secretary under President George W. Bush who had offered mild criticism of Obama’s travels, said he didn’t think Trump had crossed the line.

If the public is insistent on strict accountability, Fleischer said, “then we should go to a system where we have a nonpartisan president and a partisan prime minister. As long as you have both in the body of one POTUS, there will be the inevitable blending of politics and substance.”