As President Donald Trump baselessly claimed that all votes received after Election Day are illegal, he attacked the absentee ballots of military members and their spouses that, by law, must be counted.

In doing so, the President has endorsed a scenario where thousands of members of the military -- actively defending their country -- would be disenfranchised by having their legal votes thrown out.

"If you count the illegal votes, they can try to steal the election from us," Trump said Thursday night from the podium in the Brady Briefing Room at the White House. "If you count the votes that came in late -- we're looking at them very strongly. But a lot of votes came in late."

In every election, a lot of votes come in late -- legally -- from Americans who are overseas or located outside their states of residence, including significant numbers of absentee voters who are service members and their families.

The military has been voting absentee for two centuries -- since the War of 1812 -- and the practice was expanded during the Civil War. Military votes have been included in certified vote tallies in local, state and presidential elections. There is nothing nefarious about them.

"We're not asking for any special privilege here," retired Army Chief of Staff Gen. George Casey told CNN.

"We're just saying we think it's important for every ballot to be counted and especially those of the service men and women who serve this country. They do so much for us and they deserve to know their voices are heard."

Casey voted absentee from Iraq in the 2004 presidential election and recalls voting absentee multiple times while stationed away from Virginia, his home state of record. He is now one of many retired military leaders lending their voices to Count Every Hero, a bipartisan initiative to make sure military votes are tallied.

How the military vote works

Oftentimes, service members spend time and money clearing hurdles that their civilian counterparts are usually spared in order to ensure their ballot is counted.

"Military voters have to work extra hard to vote," said Sarah Streyder, the founder of Military Vote Coalition, a nonprofit endorsed by leading veteran and military organizations that works with service members and their spouses to ensure they can vote.

"It's a complicated registration and submission process. I know people who had to pay upwards of $50 just to overnight a provisional ballot because their official ballots never got to them in time."

Streyder ticked off multiple examples: a military spouse who lives in Texas, where her husband is stationed, but votes in her home state of Pennsylvania; spouses who live in Japan but vote in Oklahoma, Texas, Florida and Georgia.

"It's demoralizing," she said of the President's attack on legal ballots that arrive after Election Day.

Military families typically move to new duty stations every few years, including overseas. Since 9/11, service members have been more likely to deploy, often repeatedly, and they have cast ballots by the millions from war zones, overseas postings and states where they temporarily reside.

In Georgia, service members who are serving away from home, both inside the US and overseas, have until close of business Friday for their ballots to be received, as long as they were postmarked on or before Election Day.

In Nevada, ballots postmarked by Election Day can be received as late as November 10.

Some states enforce deadlines for receiving the ballots but don't require a postmark, due to the differences in how mail is processed domestically and overseas, especially on military installations or in combat zones.

Pennsylvania and North Carolina are two examples.

In Pennsylvania, these absentee ballots must be signed by the night before the election and received by November 10. A postmark is not required. In North Carolina, home to some of the most frequently deployed Army forces in the nation, that deadline is even later: November 12 with no postmark required.

As the Federal Voting Assistance Program, a voter assistance and education program created by the Department of Defense explains, "Due to varying mail pick up times, the day you 'mail' your election mail may not be the day the postal facility postmarks it. You may ask the mail clerk to hand stamp the election material so that a date is clearly visible. In certain situations a handwritten postmark and signature from you or a notarizing official may be sufficient."

Ballots 'missing' in Georgia

Trump also suggested, without evidence, that military ballots are "missing" in Georgia, insinuating the votes would tip the scale in his favor.

"Where are the missing military ballots in Georgia? What happened to them?" the President tweeted Friday.

He appeared to be referring to the 8,410 absentee ballots in Georgia that were requested by and sent to service members and their families and other overseas voters -- but have not been received back by the state.

There is nothing to indicate these votes are missing. The state of Georgia will count these ballots, as long as they arrive by close of business Friday, as they have with the 18,008 military and overseas votes already received and counted in the state.

"It's going to be more than 0 and less than 8,410," said Gabriel Sterling, the voting system implementation manager for Georgia's Secretary of State, on Friday afternoon.

Some of those ballots are likely still in transit. Many of them were requested but never cast, which is not unusual.

In 2016, election offices across the country sent 950,836 absentee ballots to military and overseas voters but only received 623,577 of those votes back, according to the Federal Voting Assistance Program.

Three percent of the ballots received were rejected, primarily because they did not arrive on time. Those not returned were presumed to have been requested but not cast.

Military support for Trump

There is also a question as to whether the votes of military and their families would benefit Trump, as the President suggested. That assumption may not be a reality.

Traditionally, military votes have skewed conservative, but the military, increasingly, is not politically monolithic.

"In 2000, the Republicans actually fought hard to get military ballots counted in that presidential election. They had an intuition, and were right, that ballots were reliably Republican in most instances," said Tara Copp, the national military and veterans correspondent for McClatchy, on CNN Right Now. "But you have a whole new generation of service members seeing 20 years of war in Afghanistan and Iraq and their political beliefs are all over the spectrum."

A Military Times poll completed in August -- a rare glimpse into the political views of active duty service members -- found support for Trump had fallen significantly, from 46 percent in 2016 to just under 38 percent. More service members said they would vote for former Vice President Joe Biden than Trump.

Beyond 2020, advocates are concerned about the long term effect of Trump's comments on the faith that service members and their families have on the absentee voting process many of them use.

"I spend every day fighting an uphill battle convincing my military friends that voting is worth it because there is this pernicious myth that absentee military ballots don't count because folks are used to having results announced before their ballots arrive," Sarah Streyder told CNN, even though military absentee ballots are ultimately reflected in the certified vote.

"Now [military families] are going to feel less confident that their votes are going to count and matter because they are being so dramatically undermined," she said.

This story has been updated with comments from retired Army Chief of Staff Gen. George Casey.