ALEXANDRIA, Minn. -- Gov. Mark Dayton, making a relatively rare appearance as a candidate Wednesday, called the troubled roll-out of the MNsure health insurance exchange the single biggest disappointment of his first term but also offered a full-throated defense of the federal Affordable Care Act.

Dayton and Hennepin County Commissioner Jeff Johnson, his Republican opponent, appeared at a general session of the Association of Minnesota Counties, held at Arrowwood Resort in Alexandria. The candidates did not share a stage, but spoke in close succession before a crowd of about 200 county commissioners from around the state.

It was something of a home-court crowd for Johnson, who has been a county commissioner since 2008. But he admitted some might not wholeheartedly support him: as a state legislator from 2001 to 2007, Johnson said, he voted for cuts to local government aid programs that the state provides to counties.

Johnson said those aid programs originated as a way to equalize funding for counties with low property wealth. "I think we've moved a long way from that, and are directing state money now to areas that probably don't need that redistribution," Johnson said.

In bringing up MNsure unprompted, Dayton was also upfront about an issue he said he knows is a sore spot for some county leaders. Many counties were closely involved with MNsure as the front-line administrators of government benefits.

"I want to thank you for the tremendous assistance you and your county staffs provided to MNsure, and I want to apologize for the excessive burdens it's placed on you, your budgets and your people," Dayton said. Calling it his biggest single disappointment so far, Dayton said, "It's got better, and it will continue to get better, but it still has a ways to go."

Despite perceptions that problems with the Affordable Care Act could cost Democrats in November, Dayton trumpeted the program without reservations.

"Over 300,000 Minnesotans have now obtained health care coverage through MNsure," Dayton said. Noting the coverage guarantee even for people with pre-existing medical conditions, he said: "One gentleman told me at the State Fair last week, quote, 'MNsure saved my life and my wife's life.'"

Johnson did not bring up MNsure in his remarks Wednesday. A spokesman said Johnson's preferred route on MNsure would be to obtain a federal waiver from the Obama administration that would let Minnesota run its own health care system; but that short of that, he would seek major changes to the way MNsure operates.

Dayton and Johnson are not scheduled to debate directly until Oct. 1 in Rochester.