Montana Gov. Steve Bullock has offered to pay back some of the costs of using a government-owned airplane to get to campaign events that coincide with state business, but some Republican lawmakers say that is not good enough and the Democrat should cover more of the bill.

Previous governors have used the plane as part of their electioneering but insist that those trips are secondary to previously scheduled official business.

Bullock said last week that he would reimburse the state $2,672 for time the pilots spent waiting while he attended campaign activities, including fundraisers, on 21 trips between last April and this month. Under new protocols issued Friday, the governor said he would compensate the state for any future use of the aircraft mixing state and campaign business.

It costs about $500 an hour to operate the Beechcraft King Air twin-turboprop, and state Rep. Brad Tschida and some other Republican legislators say paying for the pilots’ wait time is not the only cost that should be factored in to the reimbursement.

They want the governor to count the expense for his security detail and other government employees who accompany him. But they acknowledged that it is difficult to pinpoint an exact amount.

“It’s still an open question whether the governor’s numbers are accurate,” Tschida said. “It’s hard for me to determine what the extra costs are. Is it the cost for flight time? Is it the cost of the pilot? Is it the cost of the equipment? At this point, there’s a question about the reasonable value that should be reimbursed.”

The state has four pilots on its roster, including three on-call co-pilots. Their hourly wages range from $20.92 to $36.07, and they are paid more for overtime.

University of Montana political scientist Robert Saldin said there are legitimate questions to raise, but too many “people are bent out of shape” over the governor’s plane use.

“There are gray areas about where state business begins and where it ends, and when campaign business begins and where it ends. Those lines aren’t crystal clear,” he said.

The state-owned plane allows the governor to travel quickly across the large state, and Bullock has used it less than previous administrations, Bullock spokesman Tim Crowe said in an email.

Since 2013, the state-owned airplane has made at least 415 round trips from Helena, including 19 out-of-state trips to places as far away as the San Francisco Bay Area, according to flight logs supplied by the governor’s office.

In comparison, the aircraft made more than 500 roundtrip flights during the first term of Democratic Gov. Brian Schweitzer. Before that, former Gov. Marc Racicot, a Republican, reimbursed the state for trips that included campaign events and chartered a private aircraft when the purpose of those trips was primarily for campaigning.

The controversy over the governor’s plane use surfaced after The Associated Press reported last month that Bullock flew to attend a fundraiser in Billings after his official business, which included a media interview and making sandwiches with Riverside Middle School students.

The Bullock campaign will pay back $42.83 for that Feb. 10 trip, one of the 21 flights marked for reimbursement.

The AP could not independently verify whether additional trips could qualify for reimbursement because Bullock’s office has declined to release the governor’s schedule in its entirety, including campaign events. His campaign has also refused to disclose the information.

Andy Huff, Bullock’s chief legal counsel, said campaign events are non-official business not subject to public records requests.

But critics say the governor’s office needs to be more forthcoming about his schedule and use of government resources.

Republican Rep. Ryan Osmundson said if Bullock does not disclose more information about his use of the plane, lawmakers will push the issue during the 2017 session by subpoenaing Bullock’s calendar, if necessary.

“He’s obligated by law to let us know what he’s doing with tax dollars. There’s nothing private about that,” Osmundson said.