Mayors for the Freedom to Marry launched last year.
Voters in Maine, Md., Minn. and Washington to consider measures next week
The Vermont-based organization claims more than a million members
Campaigns in Maryland, Washington, Minnesota and Maine each received $250,000.
Some want president to endorse marriage, denounce N.C. and Minn. anti-gay initiatives
In symbolic move, guv vetoes 2012 ballot measure
