ST. PAUL — The campaign arm for Minnesota Democratic state senators agreed Tuesday to pay a $100,000 civil penalty to resolve an investigation over ads in the 2012 election, one of the largest fines levied in a state political case.

The penalty assessed by the Campaign Finance and Public Disclosure Board must be paid to the state’s general treasury within three months. It comes as part of a stipulation of facts in which the parties don’t admit violating campaign laws and agree to the payment to settle the dispute.

The Senate DFL and state Democratic Party operations were accused of sharing photos of 13 candidates for printed ads, which amounted to improper cooperation for units supposed to be operating independently. The state party and the candidates involved were not fined.

The complaint took more than a year for state campaign regulators to resolve. Republicans filed it a month before the 2012 election, when Democrats recaptured control of the Senate. Republicans argued that photos taken in professional shoots amounted to “active participation” by the candidates and therefore couldn’t be used in mailings that were construed as independent ads.

In its findings, the six-member board ruled that the cooperation used by the parties and candidates to produce the mailings destroyed the wall needed to keep them independent.

Minnesota Democratic Party Chairman Ken Martin said the party found it was best to see the matter resolved.

“We respectfully disagree with the Board’s conclusions regarding the Senate Caucus. However, we are pleased that they recognized, as is clearly stated in the Board’s findings, that our Party and our candidates did nothing wrong,” he said in a written statement. “Ultimately, it is best to set this distraction aside and allow our members to focus on governing.”

In his own statement, Senate Majority Leader Tom Bakk struck a similar note. “We continue to maintain that our candidates and the caucus campaign committee complied with all campaign finance laws,” he said.

Republican Party Chairman Keith Downey said the ruling comes a year too late for voters in the districts where the mailings wound up. He didn’t rule out additional requests for investigations.

“We will never know how this illegal coordination would have impacted the results in these races and ultimately control of the Legislature,” Downey said. “They cheated, they won, but at least they are being held accountable now.”

Senate Minority Leader David Hann said the ruling presents ethics

questions about the Democratic majority.

Total spending on the ads in question was $315,000, which the board determined would have exceeded the amount allowed if the ads were deemed to be direct contributions rather independent expenditures.

The penalty ranks high among punishments in state history. In 2006, a group called 21st Century Democrats was fined $190,000 for accepting illegal contributions. Late in Tim Pawlenty’s 2002 run for governor, the board fined the then-Republican nominee $100,000 and made him count another $500,000 in ad costs against his spending limit after he was found to have improperly coordinated with the state Republican Party.

Senate Democrats have a time cushion Pawlenty lacked. They don’t have to defend their majority until 2016, giving them room to raise money and minimize the impact of the fine.