For candidates in tight races and the parties that fund expensive get-out-the-vote efforts, Election Day has turned into Election Month: By Monday, voters in 34 states and the District of Columbia will be able to cast ballots in person.

The success or failure of each party’s efforts to get voters to the polls early will determine the outcome of critical contests across the nation, including the battle for control of the Senate. Both parties have invested accordingly — and early data suggests the millions of dollars they’ve poured into those efforts are paying off.

In Iowa, where Rep. Bruce Braley (D) and state Sen. Joni Ernst (R) are fighting over an open Senate seat, more than 185,000 people had cast ballots by Friday — a far higher number than had voted by this time during the last midterms in 2010. More than 782,000 Floridians have already cast their ballots this year, a little more than one-third the number who voted early in 2010.

More than 100,000 voters have cast ballots in each of these three states: California, Georgia and Michigan. Over the weekend, early-vote locations opened in Nevada and New Mexico. And on Monday, voters will be able to cast in-person ballots in Alaska, Arkansas, Idaho, Illinois, North Dakota, Texas, Wisconsin and the District of Columbia, with three states slated to begin early voting on Tuesday.

Since the first ballots of the 2014 midterm elections were cast in early September, in North Carolina, at least 1.7 million people have voted in this year’s elections, according to public records compiled by Michael McDonald, a political scientist at the University of Florida who runs the U.S. Elections Project.

Not every state that conducts early voting makes its voting statistics public. Three states — Oregon, Washington and Colorado — will conduct their elections entirely by mail this year, and ballots have already been sent in all three states. Combined with incomplete data from other states, that means the total number of votes cast probably exceeds several million.

The higher-than-expected turnout, long before Election Day, suggests early predictions of dismally low turnout might be too pessimistic.

“There’s going to be high turnout, both in the early vote and on Election Day combined,” McDonald said.

This year, Senate Democrats have invested heavily in what they call the Bannock Street Project, a multimillion-dollar effort to register, identify and turn out what they call “drop-off” voters, registered voters who tend to show up in a presidential year but “drop off” in a lower-turnout midterm.

Getting those people to cast a ballot “is absolutely critical” for Democratic hopes of keeping the Senate, said Matt Canter, deputy executive director of the Democratic Senatorial Campaign Committee. “There’s a whole lot that’s critical to our efforts to hold the Senate. There’s no question this is one of the fundamental pieces, but we’ve been preparing for this for a long time.”

After the 2012 elections, in which President Obama’s campaign used early-voting windows to run up their vote totals long before November, Republicans also have redoubled their efforts to drive their supporters to the polls before Election Day. Americans for Prosperity, the grass-roots organization attached to the network of conservative donors including the libertarian billionaires Charles and David Koch, has invested $125 million in voter mobilization projects — money that is apparently paying off.

With incomplete statistics, it isn’t clear which party has the edge overall. But it is clear that in some areas, Republicans have maintained or improved on past efforts to turn voters out before Election Day.

About 43 percent of Iowa voters who have already voted are Democrats, a sign that the party is turning out voters who might otherwise have stayed home. But around 40 percent are Republicans, a dramatic improvement over the party’s performance in 2012, when just 32 percent of the early electorate was registered Republican, and 2010, when 38 percent of early voters were Republicans.

In Florida, where Gov. Rick Scott (R) and his opponent, former governor Charlie Crist (D), have invested heavily in canvassing operations, Scott deputy campaign manager Tim Saler pointed to early statistics that show Republicans making up 48 percent of the early-vote total, compared with about 35 percent for Democrats.

That’s a nearly identical advantage to the one Scott had in 2010, when Republicans outnumbered Democrats 49 percent to 37 percent among early voters. That year, Scott won election by just over one percentage point. Many Republican-leaning counties in Florida don’t open their early voting locations until Saturday.

In 2012, when Democrats outnumbered Republicans in early voting by almost 4 percentage points, Obama won Florida by less than one point.

With numbers that tight, it’s no shock that early voting has become the latest partisan battleground in state legislatures nationwide. Between 2000 and 2010, Democratic-led legislatures expanded early-voting hours in a number of states. And since 2010, Republican-led legislatures in eight states have curtailed the number of days or hours during which early voting can take place, according to the Brennan Center for Justice.

Republicans in Missouri advanced a ballot measure that would create a six-day window for early voting — the first time voters there will be allowed to cast a ballot in person before Election Day — after early-voting supporters threatened to put a measure requiring six weeks of early voting on this year’s ballot. Voters in Connecticut this year will decide whether to remove language from the state constitution that prohibits early voting.

Methods for turning early voters out to the polls vary by state. Some campaigns rent vans or buses to drive voters to polling places. Democrats have long used Sunday church services to turn out African American voters, a practice they call “souls to the polls.” Every campaign will spend the next two weeks knocking on as many doors as possible.

The public records showing who has voted and who hasn’t help campaigns focus on turning out voters most likely to back their candidates. Campaigns and party committees have spent months, and tens of millions of dollars, identifying their supporters. Once those supporters vote, the campaigns can cross them off the target list and move on to the next potential voter.

McDonald, the University of Florida political scientist, said the focus on getting people to the polls amounted to a grand experiment, one academics have been theorizing about for years. Democrats are working to change the electorate through a concerted focus on voter mobilization, and Republicans are aiming to make up in an area where they’ve fallen behind.

“This is what the election is going to come down to: Can Democrats overcome the turnout deficits they have among their key constituencies — young people, poor people, minorities,” McDonald said. “This is a huge experiment. This is something that political scientists and parties have been doing experiments on, voter mobilization, for the last decade. And this is being done on a big scale.”