Sen. Mark Udall (D-Colo.) is banking on high turnout to help him keep his seat. (Susan Walsh/AP)

GREENWOOD VILLAGE, Colo. – Here in the Senate battleground of Colorado, the latest front in the voting wars is  the mailbox.

In other states, that fight has generally centered on laws that opponents say restrict voter access – measures, largely passed by Republican legislatures, that require voter identification or reduce the number of days for early voting.

But Colorado is  operating under a new system designed to do the opposite: For the first time this year, every registered voter will get a ballot delivered to them through the mail, weeks before Nov. 4.

The 2014 midterm elections are the first statewide contests since the Democratic-controlled Colorado legislature voted last year to make it easier to cast a ballot. The law allows residents who neglect to register in advance to sign up on Election Day itself. And it instituted all-mail elections, with ballots going out statewide 22 days before Election Day.

So Election Day, in essence, has officially become Election Month – a development that has spurred strategists on both sides to craft the biggest midterm turnout operations in state history, a high-stakes race to find and identify every possible voter.

Strategists for both Sen. Mark Udall (D-Colo.) and challenger Cory Gardner (R) say the electorate is as polarized as they have ever seen: Recent public polls show registered voters in the state evenly divided between the two men, and private polling reflects the same margin. There simply aren’t that many undecided voters to persuade.

As Chris Hansen, Gardner’s campaign manager, put it: “Everybody’s got their jerseys on.”

The fact that so many Colorado voters have  made up their mind, coupled with the new all-mail format, has changed the way the two sides will run their campaigns. While the state’s voters are used to casting their ballots early — in 2012, almost 73 percent of Colorado’s 2.6 million people voted before Election Day — the total reliance on mail-in voting has Udall and Gardner partisans focusing the lion’s share of their fall efforts on a turbo-charged ground game.

“We’re organized on the ground. We’re going to have the most focused, disciplined and well-resourced organizing campaign in the history of an off-year election,” Udall said in an interview. The all-mail election format “gives us a tool to monitor who’s voting and who hasn’t voted, and focus on both persuasion and getting out the vote.” Registered voters identified as likely supporters of a given candidate will hear from that candidate’s campaign – endlessly, incessantly, by phone and in person – until they send back their ballots. Once a county elections office reports receiving a voter’s ballot, the door knocks will stop, and the phone will fall silent.

On its face, an all-mail election that results in more people voting would seem to benefit Democrats. In Washington and Oregon, the only other states that conduct elections entirely by mail, only two Republicans have won statewide office since the introduction of that system.

Over the past two decades, Democrats nationwide have invested heavily in developing their absentee ballot and early vote programs. Their progress is especially pronounced in Colorado, where Sen. Michael Bennet (D) won a tough-fought campaign in 2010, and where President Obama beat Mitt Romney by a wider-than-expected margin in 2012, both thanks to votes banked long before Election Day. This time, Udall’s team says it has twice as many volunteers, offices and paid staff as Bennet did in 2010, and the campaign has already opened 20 field offices around the state.

A constellation of independent groups, with names like Fair Share, Mi Familia Vota and New Era Colorado, are aiding Udall by searching for new voters to register, especially younger voters, Hispanics and women, all core Democratic constituencies.

But some in the party are concerned that those groups have hit a ceiling in the last few years by registering all, or most, of the voters they can. Udall has told Democratic activists his campaign hopes to register 100,000 new voters this year, and a spokesman said the campaign stands by that number. But the number of active registered Democrats has actually fallen this year, from 896,000 active voters in January to 884,000 in August, according to the secretary of state’s office (The total number of active voters has increased by about 21,000 voters overall this year, most of whom have registered as unaffiliated with either party).

Republicans have had success building their own mail-in ballot program. Rep. Mike Coffman (R), who holds a contested seat in the Denver suburbs, was able to hold off a Democratic challenge even as  Obama won his district in 2012 by 5 percentage points, thanks in part to an aggressive absentee ballot strategy.

This year, Hansen, who ran Coffman’s campaign, is trying to take that strategy statewide. In an office park just off Interstate 25, above a Key Bank south of Denver, the Colorado Republican Party has an office with hundreds of phone lines and dozens of desks, ready to welcome swarms of volunteers for the fall push. “We want to make sure that we have our ground game at top-running speed,” Gardner said in an interview at state GOP headquarters.

Academic studies show all-mail elections don’t have much of an impact on turnout in presidential campaign years, when even the most apathetic voters are bombarded with reminders that Election Day is approaching. What is less clear is the impact of all-mail voting on off-year elections. According to some political scientists, it’s likely that the very fact that voters have a ballot sitting on their kitchen table for three weeks will mean a significant increase in turnout.

“In presidential elections, everyone knows there’s an election going on,” said Michael McDonald, a political scientist at the University of Florida. “But if you’ve got a lower statewide election, a local election or something like that, you don’t know there’s an election going on and suddenly a ballot shows up, you say, oh, I should go vote.”

Both Oregon and Washington saw dramatic increases in turnout percentage after transitioning to all-mail elections. In Oregon, turnout rose from 59 percent in 1998, the midterm election before the state went all-mail to 69 percent in 2002. Washington’s turnout rose from 64.5 percent in the 2006 midterms to 71.2 percent in 2010, the first year elections were conducted entirely by mail.

Colorado’s voter turnout is already among the highest in the nation. In 2010, 73.5 percent of voters showed up, a better performance than many states record in presidential election years.

Democrats are hoping for even higher turnout this year.

And after years of defense, they hope Colorado can be the state where the voting wars work in their favor — where an electorate expanded by the all-mail system will help Udall overcome a tough challenge. And Republicans like Gardner are hoping they’ve finally figured out a turnout formula for races that are decided entirely via the postal system, not the polling place.

The final outcome could determine control not just of a Senate seat, but of the entire United States Senate. “Much like to the victor go the spoils, to the organized go the votes,” McDonald said.