President Donald Trump racked up an estimated $17 million tab by flying on Air Force One to more than 40 political rallies in the lead up to the 2018 midterm elections, a Quartz investigation found.

Because Trump had been using the plane to stump for Republican candidates, the GOP or his re-election campaign is supposed to reimburse taxpayers for a portion of the cost. However, Quartz reports that the Trump campaign has thus far only paid the Treasury $112,667.90 in March and April for air travel expenses, or approximately 0.7 percent of the total cost. It is unclear when, or if, the campaign might issue additional payments.

Trump was highly critical of President Barack Obama during the 2016 presidential election for flying with then-candidate Hillary Clinton to a rally in North Carolina. The Clinton campaign reimbursed the government for $37,000.

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! — Donald J. Trump (@realDonaldTrump) July 5, 2016

Flying Air Force One, a retrofitted Boeing 747, costs $142,380 per hour in maintenance, fuel, onboard supplies, and other operating expenditures, according to figures provided by the Department of Defense.

Beginning on March 10, Trump relied on the plane for numerous trips to rallies throughout the Midwest and East Coast. His final rally was on Nov. 26 in Mississippi for Sen. Cindy Hyde-Smith.

Trump’s most expensive campaign-related flight cost about $1.3 million. He was traveling to Nevada in June in order to attend a rally for Sen. Dean Heller. Trump also spent $650,000 to travel to another rally in September for Heller, who would ultimately lose his re-election bid to Democratic challenger Jacky Rosen. Trump took his second-most expensive flight, at $1.2 million, in July to stump for Montana Senate candidate Matt Rosendale, who also lost his election. About half of the candidates that Trump personally vouched for ended up winning their races.

It’s not unusual for presidents to fly on Air Force One for rallies, especially because they are not allowed to travel on commercial or personal planes. However, the White House and the president’s party are supposed to ensure that taxpayers are not subsidizing travel that is partisan in purpose. Quartz reports that this may be particularly difficult with Trump, as he has been known to go off on political tangents during speeches at official events, thus blurring the distinction between partisan and nonpartisan.

During the midterm cycle, Trump used Air Force One not only as a vehicle, but also as a campaign prop. According to the Washington Post, the plane would often taxi to the rally stage and remain parked behind Trump as he spoke, serving as a visually and symbolically potent set piece. Though Presidents Obama and George W. Bush also relied on Air Force One’s majesty on occasion for rallies, the Post reports that Trump did so with particular frequency toward the tail end of midterm season.