As early voting results start to roll in, both sides are starting to see where their machines are running smoothly — and where the mechanics are breaking down. (AP Photo/Rogelio V. Solis)

After widely acknowledging their turnout operations lagged behind Democrats in 2012, early data show Republicans have made significant progress in driving their voters to the polls — but so have Democrats, who have improved their technology and tactics, driving up turnout among voters who might otherwise sit out this year’s election.

The early voting data starting to roll in adds layers of complexity to an already-opaque picture. Strategists on both sides are scrutinizing, and arguing over, far more than the number of people who have actually cast a ballot. They pay close attention to the number of people who have requested absentee ballots but haven’t returned them, the percentage of voters who did or didn’t cast a ballot in previous elections and which methods — absentee or in person — voters used to cast their ballots.

The tea leaves can be read a hundred different ways, and both sides read them to their own advantage.

Nationally, more than eight million Americans have already voted, according to early vote statistics maintained by Michael McDonald, a political scientist at the University of Florida who closely tracks early vote totals.

Democrats say their efforts to turn out voters are paying dividends in states like Iowa, where polls show a virtual tie in the race for an open U.S. Senate seat. The party is targeting 116,000 voters who are not certain to turn out — voters who either skipped the 2010 election or only recently registered.

That’s a common thread around the country: Democrats contend they are turning out a higher percentage of voters who might otherwise stay home, and that Republicans are turning out voters who were certain to vote anyway. Republicans say their efforts to get voters to the polls early frees them to spend resources turning out those who are more reluctant to vote.

In Iowa, Democrats have a slight lead in turnout among early voters: 41 percent of the 306,000 people who have showed up are registered Democrats, and 40 percent are registered Republicans. Democrats sent mailers just last week to more reliably party supporters, meaning they expect their advantage to increase over the remaining few days of early voting.

But Republicans say Iowa demonstrates just how far their party has come. In 2010, Republicans accounted for just 37 percent of early voters. As Democrats have improved their performance, Republicans have not only kept pace — they’ve narrowed the gap. What’s more, Republicans who request ballots are far more likely to return them: in 2010, almost twice as many registered Democrats as Republicans did not return ballots they had requested.

In Florida, Gov. Rick Scott’s (R) campaign has built a field operation that rivals the size of a decent presidential effort. About three-quarters of the 1.8 million people who have voted there have cast absentee ballots, which tend to favor Republicans. GOP strategists say early voting so far should favor Democrats, because key Republican-leaning counties in northern Florida didn’t open early voting stations until this weekend, but Republican voters have still made up 45 percent of those who cast votes early, compared with 38 percent of Democrats.

“They have a week left to knock down a 138,000-vote advantage by Republicans, because the whole Obama model is predicated on going into Election Day with a lead large enough that they can survive our natural election day advantage,” Tim Saler, Scott’s deputy campaign manager, said of former Gov. Charlie Crist’s (D) campaign.

Democrats don’t dispute that advantage. But they say it’s not big enough to save Scott. Democrats voted early on the first weekend the polls were open than Republicans this year, four years after GOP won that weekend. At this point in 2010, after 1.8 million people had voted, Republicans outnumbered Democrats by 15 percentage points. Today, the GOP leads by half that margin, Steve Schale, a senior adviser to former Gov. Charlie Crist (D), wrote in a memo to the media.

A tight race in North Carolina between Sen. Kay Hagan (D) and state House Speaker Thom Tillis (R) is also a campaign-year Rorschach test. Democrats are optimistic because they have a wide lead among those who have voted — 48 percent of whom are registered Democrats versus 31 percent who are registered Republicans — and because African-Americans make up a larger percentage of the electorate, 24 percent, than they did in 2010, 21.6 percent.

But Republicans say Democrats missed an opportunity yesterday, the only Sunday on which early voting stations are open. African American churches tend to drive parishioners to the ballot box — “souls to the polls,” in political parlance — but the number of Democratic votes banked on Sunday, fewer than 5,000, may have fallen short of Democratic goals.

Differences in the speed with which counties report their results also make it tough to make out the complete picture. In Colorado, where elections are being conducted entirely by mail this year, Republicans built a big advantage thanks to early votes from rural counties. That advantage shrank as Denver reported additional votes on Monday, though Republican leads in suburban Arapahoe and Jefferson Counties have Democrats nervous. The next update from Secretary of State Scott Gessler (R), which will be published Wednesday morning.

One state in which no one disputes the Republican Party’s improvement is Nevada. In 2010, with Senate Majority Leader Harry Reid (D) on the ballot, Democrats outnumbered Republicans among those who voted early by a 44 percent to 40 percent margin. In Clark County, the state’s most populous, Democrats led early voting by a nine-point margin.

Today, those numbers have more than flipped. Instead of Democrat Reid, Gov. Brian Sandoval (R) is at the top of the ticket, where he’s expected to easily win a second term against an underfunded and virtually unknown Democrat. Republicans make up 47 percent of the 135,000 voters who have cast a ballot, compared with just 37 percent who are Democrats. As of Sunday night, Republicans even led in Clark County by a slim 1,000-vote margin.

The GOP’s lead comes after Sandoval and other Republicans invested heavily in an effort to retake the Nevada state Senate, which Democrats control by a one-seat margin. It has even threatened freshman Rep. Steven Horsford (D): The outside group Crossroads GPS purchased $829,000 in television advertising attacking Horsford after early vote numbers began trickling in, in part, spokesman Paul Lindsay said, because the group was “encouraged by the early voting numbers.”

In a low-turnout midterm election environment in which few voters are undecided, strategists on both sides have spent tens of millions of dollars on massive political machines geared toward making sure their voters cast ballots. Now, with early voting underway and data flowing freely from state elections offices, both sides are starting to see where their machines are running smoothly — and where the mechanics are breaking down.