Washington — Democratic voters who have requested mail ballots — and returned them — greatly outnumber Republicans so far in key battleground states, causing alarm among GOP party leaders and strategists that President Donald Trump's attacks on mail voting could be hurting the party's prospects to retain the White House and the Senate this year.

Of the more than 9 million voters who requested mail ballots through Monday in Florida, Pennsylvania, North Carolina, Maine and Iowa, the five battleground states where such data is publicly available, 52% were Democrats. Twenty-eight percent were Republicans and 20% were unaffiliated.

Additional internal Democratic and Republican Party data obtained by The Washington Post shows a similar trend in Ohio, Minnesota, New Hampshire and Wisconsin.

Even more alarming to some Republicans, Democrats are also returning their ballots at higher rates than GOP voters in two of those states where that information is available: Florida and North Carolina.

The wide Democratic lead in mail voting so far has been the subject of urgent discussions among top GOP officials, according to people familiar with the conversations.

The margins are "stunning" - and bad news for Republicans up and down the ballot, said longtime GOP pollster Whit Ayers. While the Republican Party is focused on getting voters out on Election Day, he noted that older voters who have traditionally supported Republicans are most concerned about covid-19 infection and could choose to stay home if an outbreak arises as the election nears.

The trend has emerged after Trump has spent months assailing voting by mail, making unsubstantiated claims that it is prone to corruption and fraud — attacks that have resonated with Republican voters, polls show. State and local Republicans, fearful of losing what has long been a key turnout advantage for the GOP, spent the last few months racing to reassure voters that mail voting was safe, despite the president's rhetoric.

But GOP voter distrust in mail ballots now appears to be translating into an advantage in early voting for former vice president Joe Biden, the Democratic nominee, and for Democratic challengers in close Senate races in Maine, Iowa and North Carolina.

Democrats say they are keenly aware that the advantage could dwindle if their voters don't return requested ballots in high numbers, or if Republicans successfully challenge large numbers of mail ballots after Election Day, as they have said they plan to do.

But several Republicans acknowledged privately that there is little upside for their party in the numbers — and said they are working feverishly to reverse with a last-minute press with voters.

"It's astronomical," said one Republican strategist involved in Senate races who said he was "horrified" by the discrepancy and, like others interviewed for this report, spoke on the condition of anonymity to describe internal concerns. "You see these numbers in a state like North Carolina, and how can you not be concerned?"

The RNC is trying to close the gap, turning to Trump surrogates and family members, including the president's daughter-in-law, Lara Trump, to record calls and urge supporters to vote by mail.

The issue is of such concern that Senate Majority Leader Mitch McConnell, R-Ky., has twice met with Trump to urge him to stop talking about mail balloting "imprecisely," a strategist said. McConnell has told others he is concerned the president's rhetoric could depress Republican voters from voting by mail. And former RNC chairman and former Trump chief of staff Reince Priebus has repeatedly told others that the mail-voting gap could be the GOP's biggest challenge this fall.

A spokesman for McConnell declined to comment. The White House referred questions to the Trump campaign, which did not immediately respond to a request for comment.

Republican Party officials, meanwhile, downplayed the disparity in mail ballots so far, saying that most of their voters will show up on Election Day — and that Trump will be able to close the gap.

"We always expected to be behind at this point as Democrats have made it their mission to push for an all-mail election that brings fraud and chaos into the system," said RNC spokesman Mike Reed. "You'll see Democrats predominantly vote by mail, and our voters will come out in droves to vote in person, especially on Election Day."

The RNC and the Trump campaign are touting an extensive ground operation consisting of more than 2 million volunteers, almost 300 offices across the country and thousands of door-knockers, which officials described as a key advantage in turning out Election Day voters. They noted that Biden's team has largely foregone door-knocking, opening offices or running a traditional ground game.

Republicans are also planning to wage a massive post-Election Day legal battle to challenge ballots that are missing witness signatures, or an outer envelope, or that arrive with no postmark. Democrats acknowledge that an electorate largely unused to mail voting could make mistakes that lead to ballots getting rejected in numbers, which could make a difference in close states.

Election officials are already flagging ballots for such errors. In North Carolina, for instance, about 5,800 out of roughly 281,000 cast have been set aside so far, the vast majority because of a witness signature deficiency, according to state data. Voters still have an opportunity to remedy their errors.

"The peril we worry about is what Trump's going to try to do if there are a lot of ballots still to be counted on election night," said Terry McAuliffe, the former Virginia governor and a prominent Biden surrogate. "He's going to try to say they're fraudulent."

McAuliffe said the Biden team has made a huge push to help voters with mail balloting and encourage them to vote early, whether in person or by mail. "We've really taken it to a new level," he said.

Trump, lagging in the polls behind Biden in a slew of battleground states, has been lobbing nonstop attacks on voting by mail for months, making unfounded claims that it opens the door to rampant fraud. In fact, states that have embraced universal mail voting have documented tiny rates of ballot fraud, data shows.

More recently, the president has tried to distinguish between absentee and mail balloting, claiming that absentee ballots, which he has cast in Florida, are safe, while states that conduct universal mail voting invite fraud. In fact, the two terms are used interchangeably in most states.

Meanwhile, state and local election officials have been sprinting for months to make mail balloting easier for voters worried about coronavirus infection. Currently, after dozens of rule changes and new laws across the country, at least 84% of American voters can cast ballots by mail this year, according to a Washington Post tally.

There are already signs that this year's election is drawing out voters who did not cast ballots in the last presidential race.

Michael McDonald, a political scientist at the University of Florida who is tracking mail voting trends at his website, the United States Elections Project, noted that in some states, the number of ballots cast are already approaching 10% of the vote total in 2016. He added that turnout this fall could surpass that of four years ago before Election Day even arrives.

In North Carolina and Georgia, for instance, one in five voters who have already cast ballots didn't even vote in 2016, McDonald said. Mail ballot requests are up astronomically in dozens of states; the figure is 350% in Michigan, for instance, when compared to 2016.

In North Carolina, 17 times more people have requested ballots than four years ago; in Wisconsin, requests were up by a factor of 12, according to internal RNC data.

The fact that more Democrats have not just requested their ballots but have also turned them in is also a reversal of the trend in prior elections, McDonald said.

It means one of two things, he added: That Democrats are more enthusiastic and are filling out their ballots quickly, or that Republicans are holding onto their ballots despite requesting them, out of concern over potential fraud.

"It could be that they're listening to Donald Trump, and even though they are requesting a ballot, they are going to vote in person if they vote at all," he said.

Additionally, younger voters are a much smaller share of the voters who have already cast ballots so far than was the case overall in 2016 - leading him to believe that most young people, the majority of whom will support Biden, will vote much closer to Nov. 3 or on Election Day itself.

"I can't say with certainty that this is going to carry through," he added. "But everything that's happened over the past couple of decades, and our understanding of how people vote, is being upended in this election."

Republicans once carried an advantage among mail ballots. Ayers, the Republican pollster and a vocal critic of the president, said Trump's rhetoric on mail voting has undermined decades of GOP work in the area.

"That's what we do!" Ayers said with audible frustration. "We have made an art of tracking down people who would otherwise be reluctant to vote in person and getting them to use absentee ballots. It's part of what we've done well in the past."

He noted that the current GOP push for voters to cast absentee ballots runs counter to Trump's rhetoric.

"I've seen these appeals to likely Republican voters - 'Please apply for your absentee ballot.' But it's at the same time those voters are hearing from their president that mail voting is ripe with fraud," he said.

GOP operatives said they are hoping for a leveling of the field once early in-person voting kicks in widely, and then on Election Day.

Polls show that Trump supporters are more willing to come out to the polls on Election Day amid the pandemic. Nationally, 82% of registered voters who support Biden say they are very or somewhat worried about a family member catching the coronavirus, compared with 39% of Trump supporters, according to a Washington Post-ABC News poll conducted this month.

One benefit of banking votes early is that a smaller portion of the electorate is still subject to changing its mind. In 2016, polls showed Democratic nominee Hillary Clinton ahead in numerous battleground states at this juncture in the race, but her lead narrowed and ultimately vanished in key states as October wore on.

Similarly, campaigns can keep track of which voters have voted and cross them off their lists of people to target with advertising or get-out-the-vote exhortations. If the mail-voting trend continues to benefit Democrats, that means Biden, already enjoying a cash advantage compared to Trump, can focus his spending on a smaller universe of voters, to potentially greater effect.