Republican Greg Gianforte campaigns for Montana's House of Representatives seat vacated by the appointment of Ryan Zinke to head the Department of Interior on April 22, 2017 in Bozeman, Montana.

When the dust settles next month, it's possible that Republicans will have successfully fended off Democratic challenges in each of the competitive special election races to replace President Donald Trump's cabinet appointees.

But it's going to come with a hefty price tag for national Republican groups.