DENVER (AP) — Republicans have taken a big lead in mail-in votes cast in Colorado.

A report from the Secretary of State on Friday showed 104,000 more Republicans than Democrats had cast their ballots as the state conducts its first major mail-in election.

The figure amounts to a 9-percentage point lead with more than half the expected ballots already cast.

Colorado is a must-win state if the Democrats want to keep control of the U.S. Senate, but the early numbers suggest how difficult it might be for Democrats such as Sen Mark Udall to survive a year when Republicans are highly motivated.

New voting laws implemented in Colorado last year over objections of Republicans resulted in a ballot being mailed to every voter and allowed registration until Election Day.

So far it's Republicans who have capitalized. Of the 1.1 million votes cast through Friday, 41 percent were by Republicans, 32 percent by Democrats, and 25 percent by voters not registered with either party.

Colorado's voter registration rolls show an even split between the two major parties and people who are unaffiliated with a party.

Democrats noted that this year's voting in the state so far is disproportionately comprised of GOP-leaning groups, implying that more liberal voters have yet to cast ballots.

More than 60 percent of those whose votes have been sent are 55 or older, a segment that grew from 45 percent in the midterm election in 2010.

Democrats have been trying to turn out voters who usually skip lower-interest midterms. But it's Republicans who normally miss those elections who are voting in greater numbers this year.

The GOP even leads Democrats among voters 18 to 25, a group that has been the backbone of Democrats' dominance over the past decade in Colorado.

"They wish they were in our position right now," Michael Short, a spokesman for the Republican National Committee, said of Democrats.

Udall has lagged in the polls against Republican challenger Rep. Cory Gardner, but Democrats have been confident they can replicate their 2010 Senate win in the state and overcome the deficit through an aggressive ground game. That strategy has become a template for the party's national push to boost voter turnout and keep Republicans from netting the six seats the party needs to take control the Senate.

Chris Harris, a spokesman for Udall, said Democrats expect the Republican advantage to dwindle significantly by Election Day.

"As we've seen historically, Democrats and younger unaffiliated voters vote late," Harris said. "Things are tracking with our expectations."

Republicans have tried to match the Democrats' turnout machine in Colorado, mimicking tactics used by the Obama campaign in 2012 such as getting unreliable voters to sign cards committing to vote, pushing friends through social media to cast a ballot, and having a heavy presence of canvassers knocking on doors.

The early returns have increased Republicans' hopes of not only winning the Senate seat but also ousting Democratic Gov. John Hickenlooper.

Both parties are also targeting unaffiliated voters, a bloc that has regularly swung Democratic over the past decade. In one potential good sign for Democrats, the unaffiliated voters whose ballots have been counted are disproportionately young and newer voters, representing the party's target demographic.

Though the numbers can change, Kyle Saunders, a political science professor at Colorado State University-Fort Collins, said Democrats have work to do.

"The Democrats need to have a very good weekend," he said.

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Follow Nicholas Riccardi on Twitter at https://twitter.com/NickRiccardi .