More than two million Americans have already cast ballots for president less than three weeks before Election Day with a substantial number coming in the battleground state of Iowa.

The state, which holds six key electoral votes, is on pace to exceed the 1.5 million votes cast early in in 2008 -- a trend that is not lost on either political party.

While both parties are investing resources in early voting, Democrats' push to turn out voters -- and in particular young voters in Iowa -- could be paying off.

Early numbers, which are updated daily, show that almost 20 percent more registered Democrats than registered Republicans have voted so far.

Of the 463,219 early voting ballots requested in the state as of Thursday morning, 45 percent came from registered Democrats compared to 30 percent of registered Republicans.

Experts caution no direct link should be made between requests and how the vote might go.

"We don't know if a registered Democrat will vote for Barack Obama and we don't know if a registered Republican will vote for Mitt Romney," said Michael McDonald, associate professor government and politics at George Mason University who specializes in elections.

But return data coupled with polls that show a slight lead for Obama in Iowa, he said, can give us some clue.

The electoral makeup in Iowa looks like it did in 2008 when Obama took the state by almost 10 points, according to McDonald. So far this year, 4 percent more Democrats have voted early than in 2008. Republican numbers have only risen by 1 percent.

That's not enough for Romney if he wants to flip the state, McDonald said.

"It's not just enough to recreate the 2008 electorate. Romney needs to have improvement," McDonald said. That's assuming that Election Day returns mirror 2008 as well.

Romney's improvement might come naturally to Republicans in Linn County, Iowa, one of the Hawkeye State's most populous counties.

Tim Box, the deputy commissioner of elections there, said Democrats are more naturally inclined to vote early and Republicans are more apt to cast ballots on Election Day.

"If you paint it with a broad brush, a lot of Democrats vote absentee, Republicans go to the polls. They always start behind (in Linn County)," Box said.

He also attributed the Democratic success to an organized effort to turnout early voters. Democrats, for instance, requested more satellite voting sites for early voting and staffed them more heavily than Republicans.

There is some good news for Romney in the state since early voting requests and returns are slowly closing the gap between Republicans and Democrats, day-to-day monitoring shows.

With some "hustling" and a push at the end, Romney could make a race of it, McDonald said. He could also change the game if he is able to turn out more Republicans on Election Day than the GOP did in 2008.

And while Iowa's electoral votes could be key to winning the White House, what is true of Iowa is not true of Ohio or of North Carolina, two of the 22 states that have begun early voting.

"Each state is like a unique snowflake (when it comes to early voting)," said McDonald since the Constitution gives each state the right to administer their elections at a state level.

What this also means is that with different demographics, Democrats and Republicans have different strategies for turning out early voters in each state with different results.

What's also different is the way that states collect data, meaning that it is easier to infer conclusions from returns in states that report party registration, like Iowa.

But in Ohio, where registration is required only in primaries, the parties rely on different indicators, like returns in key strongholds.

And there is encouraging news for both: In Hamilton and Franklin counties, two urban centers around Cincinnati and Columbus respectively, early voting shows heavy turnout in the areas that helped secure the state for Obama in 2008.

The state is mailing absentee ballot applications to voters for the first time this year and McDonald said that could be affecting increased returns in rural areas, which are more heavily Republican.

In rural areas of Ohio, absentee ballots sent to each registered voter in the state for the first time in 2012 also show increased turnout.