World War I Draft Registration Cards for Maine residents appear to be mostly indexed now at FamilySearch. This is a great free alternative to the Ancestry.com collection that has been online for several years.
Three registrations were conducted between 1917 and 1918. The first was held on June 5, 1917 for men between the ages of 21 and 31. The second was held June 5, 1918 for men who turned 21 since the first registration. A third commenced on September 12, 1918 for men between 18 and 45. The registration includes cards for 24,000,000 men.