Politifact rated a Facebook post by the left-wing “The Other 98%,” that an”anti-Black Lives Matter” flag replaced the American flag behind President Trump at a campaign rally in Wisconsin as “mostly true.”
The flag in question was a thin blue line flag, which is widely recognized as a pro-police flag.
According to the Thin Blue Line USA  website the flag is flown to show support for law enforcement.
Thin Blue Line USA quickly earned its trusted name among families, departments, and communities. We are passionate sponsors of law enforcement non-profits, and make it a goal to support every family affected by the loss of an officer. We are proud to have donated over $500,000 to causes supporting law enforcement, veterans, first responders, and more.
Whether you are an officer, retired officer, or the loving family behind them, we support you. We understand the immense sacrifice you are asked to make. We stand by our products and are proud to help spread awareness of the tireless heroes who walk the Thin Blue Line.
Politifact had a different take on the flag as noted by Washington Free Beacon executive editor Brent Scher.
— Brent Scher (@BrentScher) October 30, 2020 
The basis for Polifact’s rating came from the liberal Marshall Project which wrote about the origins of the flag in which Thin Blue Line USA president Andrew Jacob admitted that he noticed the image during the pro-police response to Black Lives Matter protests, but denied that it was political or racist.
“The flag has no association with racism, hatred, bigotry,” Jacob told the Marshall Project. “It’s a flag to show support for law enforcement—no politics involved.”
Politifact’s concluded that the flag could be interpreted as being anti-Black Lives Matter and is, therefore, “mostly true.”
In terms of the Facebook claim, The image and description — that the flag has taken on greater prominence at the Trump rallies in Wisconsin — is on target. And the Thin Blue Line flag has become a prominent part of the pro-police Blue Lives Matter movement — which arose to counter the Blak (sic)Lives Matter movement.
That said, while it is possible to support both, Trump has made clear he opposes the Black Lives Matter protests — and made that opposition, and a strong law-and-order message, a prominent part of his re-election campaign. So, those attending the rally or seeing the images could easily see the flag as an “anti Black Lives Matter flag.”
We rate the claim Mostly True.