Slightly more than 20 percent of the ballots mailed out for the 2013 election have been returned, the Department of State reported Thursday morning.

That’s 622,066 out of about 3.09 million ballots that were sent to voters.

Republican registered voters continue to lead in the number of ballots returned with 258,973. Democrats have returned 192,243 ballots, and unaffiliated voters have cast 165,291.

Conventional political wisdom holds that Republican voters are more likely to vote early, and that odd-year elections are more likely to draw older voters. Early ballot returns are being closely watched because many observers believe that backers of Amendment 66, the $950 million income tax increase for P-12 education, need to boost turnout to pass the measure, especially among younger voters and people who don’t regularly vote in odd-year elections.

About half of active voters cast ballots in 2011. If that pattern holds true this year, it would mean about 1.5 million ballots will be cast. Just over 40 percent of that number has been returned.

Republican voters also have cast the most ballots in the state’s 11 largest counties, which are being tracked by EdNews in the chart below. Mesa and Douglas counties have the highest voter participation rates so far.

All ballots must be cast by mail or dropped at county voter centers by 7 p.m. on Election Day next Tuesday. Thursday was considered the last day it was “safe” to mail ballots.

The department will update its figures on Friday.

See full county-by-county list here.

Ballots returned chart