The NRA's verified Facebook page posted a photo of the sign Monday, telling gun owners, "this is a wakeup call. They're coming after us." It asks members to like and share to spread the word.
The post had been shared more than 50,000 times by Tuesday morning.
Outfront Media, which owns the billboard along Interstate 65 in Louisville, told local TV stations they are working to remove the vandalism. The billboard was tagged with "Resist 45," a group that opposes President Donald Trump.
It comes after 17 people died in a Florida school shooting, renewing debate over U.S. gun laws.
Outfront Media and Louisville police didn't return messages seeking comment.
