It’s too late for voters to send their primary election ballot by mail, Colorado’s top election official cautioned Wednesday.

Instead, voters should return their ballot by going to a drop box or voting center, according to Secretary of State Jena Griswold.

Griswold’s office also noted that all drop boxes and early, in-person voting centers are now open. 

Clerks must receive ballots by 7 p.m. on June 28. 

Voters have the option to return their ballot to more than 400 drop boxes and 130 Voter Service and Polling Centers, her office said. 

Slightly more than 10% of the state’s 3.7 million active, registered voters have returned their ballots as of June 20, election officials said Tuesday.

County clerks had received 399,466 ballots, according to figures released by Secretary of State Jena Griswold.

Democrats are so far returning their ballots faster than Republicans, who are also slightly lagging participation by unaffiliated voters. That’s despite voters in the GOP primary facing more contested races, including statewide primaries for U.S. senator, governor and secretary of state.

Of the ballots received through 11:30 p.m. on Monday, 137,028 were cast by Democrats, 128,898 were cast by Republicans, and 133,484 by unaffiliated voters, most of whom received both major parties’ primary ballots in the mail, but can only fill out and return one of them.

In total, voters have cast 170,452 ballots in the Democratic primary and 168,594 in the Republican primary – including ballots cast by registered Democrats and Republicans, respectively, and unaffiliated voters participating in the two parties’ primaries.

Unaffiliated voters have so far opted to vote more Republican ballots than Democratic ones, though, with 39,696 filling out the GOP ballots and 33,424 voting in the Democratic primary, according to the preliminary report. Another 60,364 ballots have been received from unaffiliated votes but haven’t yet been processed and sorted.

Reporter Ernest Luning contributed in this story.