Public Service Commissioner Kevin Cramer (R) could upset his party-backed opponent in the Tuesday primary for North Dakota’s open at-large seat, a new poll suggested.

Cramer led fellow Public Service Commissioner Brian Kalk, 60 percent to 21 percent with 19 percent undecided, in a Mason-Dixon poll sponsored by the local Valley News Live/KFYR-TV. The poll surveyed 625 likely voters from June 4 to 6. It had a margin of error of 4 percent.

A Cramer victory would make history in North Dakota GOP politics. It’s rare for anyone to mount a significant challenge against the GOP’s endorsed candidate in a primary — let alone win the GOP nod.

Earlier this year, party delegates endorsed Kalk for the open House seat at the state convention. Rep. Rick Berg (R-N.D.), who is running for Senate, also told Roll Call that he’s backing Kalk.

But national conservative activist groups quickly coalesced behind Cramer. FreedomWorks and the Club for Growth, active on behalf of other anti-establishment candidates in several GOP primaries across the country, have backed Cramer and spent on his behalf.

Last month, the Fargo Forum sponsored a GOP primary poll that showed Cramer leading Kalk by a smaller margin, 38 percent to 25 percent. In that poll, a larger portion of the respondents — 37 percent — were undecided.