Republican candidate for governor Jeff Johnson blasted Gov. Mark Dayton on Tuesday for what he called "breathtaking incompetence," after it was revealed that the insurance company that sold more policies on MNsure than any other is pulling out of the exchange.

MNsure officials confirmed Tuesday that PreferredOne, the choice of nearly six in 10 consumers who have bought plans on the exchange, would no longer participate. Insurers are expected to release their 2015 MNsure rates in early October.

"Mark Dayton was desperate to be the first governor in the country to implement Obamacare in Minnesota through MNsure," Johnson said at a Capitol news conference. "He got to handpick his board and handpick his staff and it has been an unmitigated disaster since day one."

Johnson suggested that PreferredOne pulled out because it was offering artificially low rates on its plans under pressure from Dayton's administration. Jeremy Drucker, spokesman for Dayton's re-election campaign, called that ridiculous.

"Of course, administration officials encouraged insurers on MNsure to provide the lowest rates possible to the people of Minnesota," Drucker said. "However, the companies were solely responsible for the rates they decided to offer."

Dayton has owned up to MNsure's struggles. He apologized to consumers who struggled to buy coverage through the glitch-ridden website, and earlier this month he called MNsure's troubled launch the single biggest disappointment of his first term as governor.

Johnson said if elected, his first goal would be winning a federal waiver that would allow Minnesota to pull out of conforming to the Affordable Care Act. If that's not successful, Johnson said, he would seek to replace MNsure's board of directors and its leadership staff. He also said he'd try to increase competition among those companies selling plans on the site.

Drucker predicted that if Johnson is elected, he would seek changes that undermine MNsure's mission of boosting insurance coverage rates.

"This would be devastating to Minnesotans," Drucker said.