Democrats are building a big lead among mail-in and early ballots, but Republicans could dominate the vote on Election Day

• Democrats have a large advantage in the mail-in ballots returned so far, but it's not enough to guarantee them a victory.
• Due to President Trump's aggressive campaign discouraging mail voting, Republicans are far more likely than Democrats to vote in-person on Election Day.
• The United States could also be on track for record voter turnout this year.
• Data from the US Elections Projects shows that over 18 million voters cast a ballot as of October 15, a number that makes up over 13% of the total votes counted in the 2016 presidential election.

Democrats currently hold a substantial advantage in the early mail-in ballots cast so far and the United States could be on track for record voter turnout this year, but that doesn't mean the Democratic presidential nominee Joe Biden is guaranteed a win on Election day.

Nineteen days out from the election, all states have begun sending out mail ballots to voters who requested them, and in-person early voting is underway in 25 states.

As of Thursday, October 15, over 18 million voters have voted by mail or early in-person in 40 states and the District of Columbia, according to a database maintained by the US Elections Project at the University of Florida, a rate that far exceeds the number of ballots cast at the same point in the 2016 cycle.

Due to the COVID-19 pandemic, most US states have expanded either voting by mail, in-person early voting, or both.

But shortly after the pandemic exploded in the United States, President Donald Trump and his campaign began waging a public relations and legal battle against voting by mail, falsely claiming that expanded mail voting leads to increased voter fraud and is not secure.

As early as July, public opinion polls and mail ballot request trends indicated that Trump's rhetoric was having an impact in shaping how the president's base plans to vote — even in states like Florida, where the state's expansive vote-by-mail rules have benefited older Republican voters in recent years.

In the 15 states that are reporting returned mail ballots by party registration, the data compiled by the US Elections Project shows that Democrats requesting mail ballots at higher rates than Republicans and returning their ballots at higher rates.

As of Thursday, 56% of mail ballots returned came from registered Democrats, 24% were cast by registered Republicans, and 20% were cast by voters with no party affiliation.

The swing states that provide a breakdown of mail ballots returned by party registration -- Florida, Iowa, North Carolina, and Pennsylvania -- all show registered Democrats leading registered Republicans by double-digit margins in mail ballots returned.

But while the Democrats' current lead is a certainly promising start, it doesn't necessarily mean that the party's candidates, including Biden, will win big in November.

Because of the sharp partisan divides in how Americans plan to vote, it's likely that the in-person vote on Election Day will lean Republican and could be extremely lopsided towards Republicans in some areas. A recent national poll conducted by The Economist and YouGov found that 45% of Trump voters plan to vote in-person on Election Day compared to just 18% of Biden voters.

"With so many Democrats voting by mail, and with Trump supporters listening to him undermine mail ballots, I would not be surprised if the in-person early voting is unusually strong for Republicans," Michael McDonald, a University of Florida professor who runs the US Elections Project wrote on Sunday. "Election Day should be bright ruby-red, and we'll see where the balance tilts when all is said and done."

McDonald previously noted that, regardless of whether the current early vote trends are predictive of the final outcome, Democrats have the key advantage of being able to more effectively target voters to turn out in the final days before November 3.

Voters casting their ballots before November 3 eliminates the chance that weather, last-minute emergencies, or a spike in COVID-19 cases could dissuade them from the polls on Election Day itself.

It also means that Democratic campaigns can now cross those who have already cast their ballots off their lists and focus their energy on turning out those who haven't already voted.

Aside from the partisan considerations, the United States could be on track to hit two big milestones: the first presidential election in which the majority of the electorate casts their ballots before Election Day and a new record for total voter turnout.

In 2016, about 17% of the electorate voted early in-person, 23% voted absentee or by mail, and 60% voted in-person, according to the US Election Assistance Commission. In 2020, that dynamic could be reversed, with the majority of the electorate now poised to vote before Election Day.

So far, the total number of votes cast accounts for over 13% of the total votes cast in the 2016 presidential election.

Over a dozen states, including the key battlegrounds of Florida, Georgia, Iowa, Michigan, and Wisconsin, have already reached or surpassed 20% of 2016 turnout, the US Elections Project shows.

As McDonald has pointed out, it's still unclear exactly how much of the current early and mail vote represents a genuine increase in turnout over 2016 levels, or how much is due to people who would have voted in-person on Election Day casting their ballots earlier instead, a phenomenon known as cannibalization.

But Tom Bonier, a strategist and data analyst at TargetSmart, has argued that the current levels of early voting and the demographic breakdown of those voting early are indicating a sizeable increase in turnout beyond what can be explained by Democrats cannibalizing their votes. Bonier tweeted on Wednesday that Democrats currently hold an 18-point advantage among those who did not vote in 2016.

If the US does set a record for voter turn out, and certain demographics like Black and young voters cast their ballots in high numbers, the Democrats could run away with the election. But because of the uncharted territory the country is in with the coronavirus pandemic and the lack of definitive final data, it's too soon for Biden and his party to let their guard down yet.