In response to Juneau's announcement, Montana Republican Party Chairman Jeff Essmann criticized Juneau as a liberal bureaucrat whose career has been marked by implementing Common Core educational standards in the state.

"Denise Juneau is a classic example of a career politician in desperate need of employment by the government," Essmann said in a statement.

Juneau first floated the idea of a campaign two weeks ago.

She has been the state's public schools superintendent since 2009, and term limits prevent her from running again. She said she could have chosen to go into the private sector, but she enjoys public service and was concerned that Zinke was not making the right choices for Montana.

She cited as examples his votes to block funding Planned Parenthood, against a recent budget agreement and for two cybersecurity bills she said would invade personal privacy.

Juneau also pointed to Zinke's announcement last month that he was considering running for House speaker if Rep. Paul Ryan, R-Wisconsin, did not step up to replace the outgoing speaker, John Boehner.

"That's not what our state sent him there to do," Juneau said.