Republican JD Vance is alleging Democrat Tim Walz is guilty of "stolen valor" -- an explosive allegation that supporters of Walz say goes too far.
The allegation also has legal implications. It's against federal law to lie about military service to try to defraud people.
While there's no evidence that Walz is guilty of such a crime, there are questions about how Walz has talked about his service, including claims that he carried a gun "in war," when he never saw active combat.
The Harris-Walz campaign issued a statement saying that "in his 24 years of service, the Governor carried, fired and trained others to use weapons of war innumerable times. Governor Walz would never insult or undermine any American's service to this country -- in fact, he thanks Senator Vance for putting his life on the line for our country. It's the American way."
On Thursday, while speaking with reporters in Detroit, Michigan, Harris said, "Listen, I praise anyone who has presented themselves to serve our country. And I think that we all should."
Here’s what to know:
Walz retired from the Army National Guard as the crisis in Iraq was growing to run for political office.
By 2005, Tim Walz had logged 24 years with the Army National Guard and earned the rank of command sergeant major -- the highest enlisted rank for his unit and one with considerable responsibility.
It was possible his unit would be sent to war, as Pentagon officials hoped the presence of U.S. troops in Iraq would protect the new coalition government and prevent a civil war.
Walz, a vocal critic of the Bush administration's war policies and father of a 4-year-old girl at the time, opted to file his retirement papers that May and run for public office instead.
Walz won, successfully flipping a Minnesota House seat from red to blue for the first time in years and becoming the highest-ranked enlisted veteran to serve in Congress.
According to records by the National Guard, the 1st Battalion of the 125th Field Artillery received an alert order on July 14, 2005, – two months after Walz retired. The mobilization order came in August and the unit mobilized in October.
Joseph Eustice, another retired command sergeant major who served with Walz, tells ABC News that while there was speculation of a deployment around that time there was no firm indication that Walz’s unit would be sent to Iraq until that July alert order.
Eustice says he remembers Walz struggling with the timing of wanting to serve as a lawmaker but also avoiding asking for a deferment so he could do so.
"He had a window of time. He had to decide. And in his deciding, we were not on notice to be deployed. There were rumors. There were lots of rumors, and we didn't know where we were going until it was later that, early summer, I believe,” Eustice told ABC News.
Vance said Walz as a command sergeant major at the time shouldn’t have allowed his unit to deploy without him.
"I think it's shameful to prepare [a] unit to go to Iraq to make a promise that you're going to follow through and then to drop out right before you actually have to go. I also think it's dishonest,” Vance said.
White House Press Secretary Karine Jean Pierre said his record as someone who enlisted at age 17 and spent more than two decades in the Army National Guard "speaks for itself.”
Walz served as command sergeant major but didn't stay in that role long enough to keep it in retirement.
There's no evidence Walz broke any rules by retiring when he did. After more than 20 years of service with the National Guard, a person can retire at any point, regardless of where they are in their contract, according to the National Guard.
Walz had joined the Nebraska National Guard at age 17, later transferring to the Minnesota National Guard, spending a total of 24 years in the service.
According to experts, Walz's retirement would have to have been approved by his commander. Officials say retirement requests can be denied if a Guard unit is mobilized, though the ultimate decision is up to the unit commander.
What is noteworthy is that Walz still uses the rank of command sergeant major on his website.
Walz indeed achieved that rank in service in September 2004. But he would have had to serve in that particular role for three years to retire as one officially, according to the National Guard.
"After 24 years in the Army National Guard, Command Sergeant Major Walz retired from the 1-125th Field Artillery Battalion in 2005,” according to the governor's website.
Walz said he carried a gun "in war,” but he also has acknowledged he was never in direct combat.
In a video clip tweeted out by the Harris campaign on Tuesday, Walz tells an audience that he carried guns "in war” while trying to make the case for restrictions on gun access.
"We can make sure that those weapons of war, that I carried in war, are the only place where those weapons are at," Walz said.
Walz would have carried a gun during his service when the U.S. was at war following the 9/11 terror attacks, including a deployment to Italy in 2003 in support of Operation Enduring Freedom.
But there is no evidence that Walz was ever engaged in active combat, and he has acknowledged as much, according to a 2018 interview with Minnesota Public Radio.
"I know that there are certainly folks that did far more than I did. I know that," Walz said. "I willingly say that I got far more out of the military than they got out of me, from the GI Bill to leadership opportunities to everything else."
Still, Vance insists Walz's comment about carrying a gun "in war” and not "during war” is tantamount to stolen valor.
"What bothers me about Tim Walz is the stolen valor garbage. Do not pretend to be something that you're not,” he said.
Vance is a former Marine who served in Iraq but wrote in his memoir that he was "lucky to escape any real fighting.”
"I served in a combat zone. I never said that I saw a firefight myself, but I've always told the truth about my Marine Corps service. That's the difference," Vance said Wednesday.
ABC News' Isabella Murray and Alex Presha contributed to this report.
