Minnesota's reputation as a squeaky-clean state is coming under attack.

The Minnesota Majority, a conservative group that has investigated the state's voting system and campaigned for a strict photo ID requirement, put up a billboard near Elk River that says Minnesota is "number one" in voting fraud.

Dan McGrath, executive director of Minnesota Majority, referred to a study the group did of convictions of felons voting illegally in the 2008 election. At the time of the report in October of 2011, there were 113 convictions.McGrath said the number is now more than 200, and that is the most for any state for one election since Missouri in 1936.

"Minnesota's election system has been hailed as 'exemplary' and a 'national model,' " McGrath said in an email message. "But I don't think leading the nation in proven cases of voter fraud is the kind of example we want to set."

Felons can legally vote in Minnesota once they have finished all the terms of their sentence, including probation and parole, and have had their civil rights restored. But voting by felons who are not incarcerated -- but not yet cleared to vote -- has been recognized as a problem.

Elections officials say because most felons do have current IDs, a photo ID requirement would not solve the problem. A state task force has studied possible solutions, including North Dakota's, which is to allow felons to vote once they are released from prison or jail, even if they remain on probation.

Greta Bergstrom, a spokesman for TakeAction Minnesota, a liberal group that is leading the drive against Photo ID, said county attorneys vigorously investigate and prosecute any evidence of illegal voting.

"Their billboard is as misleading and alarmist as the photo ID constitutional amendment they placed on the November ballot," she said, adding that the amendment "wouldn't address or solve the kind of elections issue Mr. McGrath is promoting without substantiation. But we do know it would radically change Minnesota's elections system which is, in fact, regarded as number one in the nation."

This year, the Legislature approved a proposed constitutional amendment that would require all voters to show a valid government-issued photo ID. The amendment would also create a new system of two-step provisional voting for those without proper IDs, and would tighten eligibility requirements for some voters. It is headed to voters in November and is now the subject of a state Supreme Court challenge.

There were 2.9 million voters in the 2008 general election, but the final margin in the U.S. Senate race was minuscule -- 312 votes for Democrat Al Franken over Republican Norm Coleman. That decision was arrived at after an 8-month recount and court battle.

Minnesota Majority argued that their research, in matching voting lists with conviction data, showed far more felons who appeared to have voted illegally in 2008 but who could not be prosecuted, because the law requires that they knowingly violated the law.

McGrath said this is the only billboard up right now but others may be added.