Voter registration is at an all-time high in Maine, with Democrats gaining most of the ground at the expense of Republicans and unenrolled voters, according to data from the Secretary of State’s Office. Unenrolled voters still make up the biggest bloc in Maine, about 36 percent, but Democrats have registered more than three times as many additional voters as Republicans since this time last year. According to data released Thursday, the number of registered Democrats has jumped by nearly 16,000, from 309,100 last year to 324,820 so far in 2016.