TALLAHASSEE — Even as Florida attracts hundreds of new residents every day, the state's pool of active voters is actually shrinking.This paradox is easily explained. All 67 counties must periodically scrub the voter roll to make it more accurate and to be sure voters live where they say. Counties can't do that close to an election, so they do it in non-election years.Turns out, that's good news for Republicans and bad news for Democrats.In Florida, a revolving-door state where people are constantly coming and going, the roster of active voters keeps changing. The voter roll expands in a presidential election year, when political parties are aggressively signing up voters, and it shrinks the following year, only to grow again, then shrink, like an accordion.The people who run elections in Florida, the 67 county supervisors of elections, want those who are eligible to vote.Their scrubbing of the voter roll aims for missing voters. Known as list maintenance, it is mandated by the state Legislature.Many "missing" voters have simply moved away but didn't tell the post office where they were going. Those who can't be found after two official mailings, the second one with prepaid postage, must be listed as inactive.Those listed as inactive can still vote if they show up at the polls or call an elections office. But they are in a civic purgatory: They do not count on the official voter roll, and they won't get sample ballots and reminders to vote, making it less likely they will keep voting.Florida does not permanently remove anyone from the voter roll unless they die, are convicted of a felony, are judged mentally incapacitated or are not citizens.Voters also can be removed if, after being classified as inactive, they skip voting in two consecutive federal general elections. That takes up to four years.In the nation's largest swing state, the scrubbing of the roll results in more Democratic voters being listed as inactive than Republicans.That statistical reality — in a state where the 2016 presidential election and the past two races for governor have each been decided by about 1 percentage point — suggests that Democrats have a larger group of voters who are on the move or who are not engaged politically."My party needs to have a year-round voter registration effort," said Steve Schale, a Democratic political strategist who played a key role in Barack Obama's victory in Florida in 2008. "We keep seeing these peaks and valleys in voter registration. We're not as engaged as Republicans, and a lot of our folks drop off. There has never been a year-round effort to sign up new voters."The best case study of the Democratic Party's disadvantage can be found in Hillsborough, the largest swing county in the state. It had 46,264 Tampa-area voters moved to inactive status in the first six months of the year.That's by far the most of any county and is more than twice as many as the next highest county, according to state records."We're doing a really good job," said Hillsborough Supervisor of Elections Craig Latimer. "I'm a firm believer in having as clean a voter roll as possible."Rep. Blaise Ingoglia, R-Spring Hill, chairman of the Republican Party of Florida, said the off-year fluctuations in registration are not new."The Republican voter roll appears to be much more stable than the Democrats' voter roll," Ingoglia said.Individual voter data from Hillsborough requested by the Times/Herald shows that more than twice as many Democrats (18,514) were moved to inactive status as Republicans (9,140) in the first six months of 2017.Every two months, Latimer said, Hillsborough uses the Postal Service's national change of address registry to check for voters whose address may have changed.Other counties may do that exercise once a year.During the same six-month period, Pinellas moved 19,913 to inactive. Miami-Dade moved 11,462 to inactive status through June 30 and Broward moved 15,214 voters, based on reports filed with the Division of Elections.Counties also can find missing voters when they send out sample ballots before an election, when they notify voters of address changes of polling places or send out new voter information cards because of changes to political boundaries through redistricting.But voters who have moved and who don't provide a forwarding address to the Postal Service are difficult for elections officials to find.The shrinking of the voter roll in an odd-numbered year also affects Democrats more than Republicans in Florida.Statewide, between Jan. 1 and Aug. 31, the roll of active voters shrank by 114,000, to 12.8 million.That's the net difference after adding newly registered voters and subtracting inactive voters and those who have been removed from the rolls because they died, were convicted of a felony or asked to be removed from the rolls.The statewide total of Republicans shrank by about 27,000 in those seven months, and the statewide total of Democrats shrank by about 82,000.Democrats still hold an edge in total registration at 37.6 percent to the GOP's 35.4 percent.Voters having no party affiliation or who belong to a minor party make up 27 percent.Through Aug. 31, the Democrats' registration edge stood at 275,330 out of nearly 13 million voters, compared to 330,428 at the start of the year.Palm Beach County Supervisor of Elections Susan Bucher said the frustration of not finding transient voters would be a lot easier if Florida joined a multi-state consortium, in which participating states swap voter registration information.Gov. Rick Scott's administration has pushed to join the consortium."We're the state that people move to," Bucher said.Contact Steve Bousquet at [email protected] and follow @stevebousquet.