By Tom Scheck, Minnesota Public Radio News

ST. PAUL — Gov. Mark Dayton defended his Commerce Department Friday, saying officials there accurately calculated that the average cost on health plans offered through MNsure would rise only 4.5 percent next year.

“I maintain — and we said it honestly and straightforward — if you take the four plans that are in it this year and next year and it’s going to be 4.5 percent,” Dayton told reporters following a campaign stop.

Republican challenger Jeff Johnson, however, wasn’t letting up on his criticism that Dayton’s people low-balled the 2015 increases. The claims were bolstered in recent days when outside analysts suggested that a properly weighted average calculation would put the increase at more than 10 percent.

Johnson, who’s been pushing to make MNsure a campaign issue, said the exchange’s botched rollout and costs have been a disaster for Minnesota and that Dayton is “lying” about the size of the cost increases for 2015.

If elected governor, Johnson said he would seek a waiver from the federal government to opt out of the exchange and build a new state program.

Dayton conceded the rollout of the MNsure website last fall was the biggest disappointment of his first term. But the governor argued that MNsure is affordable and doing good things and that Johnson hasn’t offered any concrete alternative.

“You can slice and dice the numbers to come up with different results,” Dayton said. “We still have the lowest rates of any state in the nation from the last reports that I have seen.”

Dayton also said he did not personally pressure Golden Valley-based insurer PreferredOne to lower its rates last year. PreferredOne had the largest number of private enrollees in MNsure and offered the lowest rates last year but opted not to sell policies on the exchange this year.

Officials at the health plan aren’t commenting, but Dayton called it a “business decision” and emphasized that the commerce commissioner can negotiate with health plans after they propose their initial rates.

While Republicans were targeting Dayton on MNsure, they were still explaining a problem of their own.

The Republican Party of Minnesota decided yesterday to change a TV attack ad against Dayton after a Starbuck woman complained about the ad’s use of her dead grandson’s picture.

The ad featured Eric Dean, a 4-year-old killed in 2013, to try to blame Dayton for a breakdown in child protection services.

Party Chairman Keith Downey declined to be interviewed. But he but told WCCO Radio Friday morning that he’s responsible for the ad. Downey said he apologized to the boy’s grandmother, Yvonne Dean.

Johnson said he doesn’t know if the backlash over the ad will hurt his campaign, but he said he’s thankful the party changed it.

“I thought it was a terrible ad. I’m not going to say anything more than that,” Dayton said.

Dayton and Johnson are scheduled to debate once more on Oct. 31 in St. Paul.