Gov. Mark Dayton couldn't keep the voter ID amendment off the November ballot, but he said on Monday he will do whatever he can to convince voters to defeat it and kicked off his pledge with a symbolic veto of the amendment.

"Although I do not have the power to prevent this unwise and unnecessary constitutional amendment from appearing on the Minnesota ballot in November, the Legislature has sent it to me in the form of a bill," the governor wrote in a letter to the speaker of the House, three days after legislators passed the legislation by party-line votes. "Thus, I am exercising my legal responsibility to either sign or veto the amendment. I am vetoing the amendment and its title; I urge Minnesotans to reject it in November."

Republicans quickly shot back, calling the governor's "mock veto" an attempt to mislead voters.

"The governor's action today misleads voters by suggesting the governor has the authority to override the Legislature's right to place a question on the ballot," said Senate Majority Leader David Senjem, R-Rochester.

The battle over whether Minnesotans should be required to show photo identification to vote now shifts out of the Legislature and to the state's airwaves, websites and water coolers.

For the next seven months, both sides will be working to convince voters that the proposed ballot amendment is either a common-sense measure to prevent election fraud or a partisan maneuver that could disenfranchise thousands of Minnesotans.

"We know we're the underdogs; we recognize that," said Dan McGrath, executive director of Take Action Minnesota, one of the grass-roots groups rallying to oppose the photo identification amendment. "We have to organize a strong and united campaign to help Minnesotans understand that any manipulation of the constitution for narrow political gain is a bad idea."

Within the week, McGrath said, a broad coalition of groups will step forward to spearhead opposition to the ballot amendment, organized through the site.

"We're going to have to run a very broad and sophisticated campaign," McGrath said. "Whether we're knocking on your door or sending you e-mails, we're going to be out there, getting the message out."

Enshrining photo identification in the state Constitution has been one of the more contentious issues this legislative session. Floor debates dragged on late into the night and early morning. There were allegations of voter fraud, countered by warnings that seniors, students and the disabled would be disenfranchised and that the cost of obtaining a photo ID was creating a throwback to the days of poll taxes.

Polls have shown widespread support for the idea of requiring proof of identification to vote. At his news conference, Dayton accused the GOP of organizing its legislative agenda around "whatever polls well."

The pro-amendment Minnesota Majority is planning its own get-out-the-vote campaign. The head of that organization is also named Dan McGrath. That McGrath said his group is planning pro-voter ID campaign spots, a fall rally and a wide mobilization of volunteers through its website.

"The opposition is willing to spend a lot of money opposing this," Minnesota Majority's McGrath said. "Clearly, we're going to be working to maintain the support level in favor of voter ID."

More and more states are debating photo identification for voters. Eight states already have laws on the books or amended to state constitutions, and, according to the National Conference of State Legislatures, there are photo ID proposals pending in 30 more.

Jennifer Brooks • 651-925-5049