Trump supporters held up Clinton "target practice" posters at a rally in Florida Monday, with a bulls-eye framing her face.
Two days earlier in Virginia Beach, one Trump backer hoisted a plastic Hillary Clinton head on a stick, while others waved target signs. And several weeks ago, two armed Trump supporters protested outside the congressional campaign office of a rural Virginia Democrat, in what they said was a gesture of solidarity with closet supporters of Trump.
Federal and state law enforcement officials say such incidents have heightened their concerns about violence in the final two weeks of the long and bitter Presidential campaign, and well beyond that if Donald Trump loses and refuses to accept the vote as legitimate.
"There is a motivated army of 'fingers in their ears' supporters of his who believe he's giving them license to behave badly, and license not to except the findings of the 9,000 bipartisan polling jurisdictions around the country," one senior federal law enforcement official told NBC News.
"It gives me concerns about violence," that official said, not just on Election Day but before and afterward. Speaking on the condition of anonymity, he confirmed that local, state and federal authorities and even multi-agency Fusion Centers are on the lookout for signs of trouble.
The senior official said authorities have been concerned for months about violence, especially after one Trump surrogate talked in August of a post-Election "bloodbath" if supporters sensed any kind of voter fraud, and another said Clinton "should be put in the firing line and shot for treason." Trump himself ignited a controversy Aug. 9 when he hinted that pro-gun activists might take into their own hands the issue of Clinton appointing pro-gun control justices to the Supreme Court.
Those fears increased exponentially last week, the official said, when Trump refused to support the election results during the final Presidential debate, and then doubled down on his calls for his supporters to act as freelance election monitors at the polls.
Trump has insisted his remark about Second Amendment supporters was taken out of context, and his campaign has denied that he is intentionally trying to incite violence of any kind.
Local law enforcement officials in several swing states told NBC News they did not yet have plans to alter their usual Election Day security procedures.
But the FBI and Department of Homeland Security are monitoring the situation, officials say, going so far as to talk to certain individuals who have somehow risen above the din of those espousing violence. They say that there is significantly more concern about violence than in past campaigns, as Trump -- dropping in many polls -- has issued increasingly strident accusations about dead and illegal voters and a "Crooked Hillary" rigging the election.
A second senior federal law enforcement official told NBC News on Tuesday that while authorities so far have not picked up "anything of substance, we're watching very closely."
"It's a genuine concern," that official said, "but we haven't seen anything that goes beyond rhetoric."
One internal Virginia state "Joint Special Event Assessment" issued Oct. 24 warned of a wide array of potential threats, including U.S.-based supporters of the Islamic State terror group attacking "soft targets, including polling places and other election-related venues … because they are vulnerable, are easy to exploit, and involve mass gatherings of people."
The assessment cited a recent ISIS propaganda video that included what appeared to be media footage of a polling site in Berryville, Virginia.
But that assessment, obtained by NBC News, also cited concerns about violence sparked by one of the most rancorous election campaigns in recent history.
"Voter fraud concerns may cause intimidation, tension, and violent confrontations among voters, poll workers, and poll watchers at or near polling places," the Virginia assessment said.
The assessment didn't cite Trump's calls for his supporters to monitor voting on Election Day. But it noted that, "Some poll watchers' presence may intimidate voters and poll workers," and that changes in federal election law have reduced the number of federal election observers stationed inside polling places to guard against voter intimidation.
The Virginia Fusion Center cited a two-person armed protest in front of a Fluvanna County, Virginia campaign office, reports of campaign signs being set on fire in Fredericksburg and reports of campaign sign thefts in Prince William County, as well as a firebomb attack on a Hillsborough, N.C. campaign office "and similar incidents that have occurred in the United States in recent days."
Media reports say two Trump supporters protested outside the Fluvanna County campaign office of Democrat Jane Dittmar two weeks ago to protest Clinton, unite like-minded Trump supporters and to make a statement after the Trump campaign's reported decision to write off Virginia. "He might be pulling out, but we're not pulling out, and I'm gonna stand my ground and speak out for what I believe in," protester Daniel Parks told the Huffington Post. Both men were carrying lawful sidearms but witnesses said they exposed them in an intimidating manner.
The Virginia Fusion Center report also warned about possible trouble by the New Black Panther Party. During the 2008 Presidential election, NBPP members were observed standing outside a Philadelphia polling place wearing military clothing with batons on their belts. In 2012, members were observed "guarding" polling places in Philadelphia and Cleveland, and prior to the 2016 Republican National Convention, the group's chairman said, "If it is an open state to carry, we will exercise our Second Amendment rights."
The senior federal law enforcement official would not discuss specifics about how authorities are on the lookout for signs of trouble, but confirmed that said some local, state and federal authorities are quietly watching known agitators to try to anticipate potential problems. He stressed that authorities also worry that being too aggressive would create the perception of undue interference and violations of individuals' right to Free Speech.
Authorities are especially concerned about the "I'm a patriot and if it comes to it, I'll grab my weapons and defend my flag" Trump supporters, the official said. But they are also worried about the unpredictability of "the rise of the unsettled masses. It is really disturbing."
Branden Belloni, a Trump supporter at the Florida rally, told NBC News' Hallie Jackson that he didn't think his Hillary bulls-eye poster was threatening, or sent the wrong message to his 8-year-old son, who was standing next to him. He had pulled the third-grader out of school to attend the event.
"I'm not a violent person," he said, "I don't teach my son to be violent. You know … it's a humor thing."