At the Republican National Convention in 2016, after Mr. Trump had captured the nomination, David Clarke, the Black sheriff of Milwaukee County, stood before the crowd, wearing a thin blue line flag pin on his uniform, and railed against the “anarchy” of the Black Lives Matter movement and the “collapse of social order.”

By the time Mr. Trump’s 2020 re-election campaign rolled around, the Black Lives Matter movement was resurgent in the wake of the death of George Floyd, given fresh urgency by the Trump presidency and a harder edge by the dislocation caused by Covid. The Trump campaign cast the Democrats as enemies of law and order who sought to incubate riots in American cities and chaos at the border.

At a rally in Waukesha, Wis., the thin blue line flag took center stage, hanging above the crowd behind the lectern at which Mr. Trump spoke, in the position traditionally occupied by the American flag. In Macon, Ga., it hung vertically behind the stage in a diptych with the Stars and Stripes. At a rally in Bullhead City, Ariz., it festooned the bleachers flanking the podium, while an enormous version, held aloft by a crane, waved high above the crowd. From the stage, Mr. Trump claimed that while the Democratic ticket stood with “rioters and vandals,” he stood with “the heroes of law enforcement.”

At the Republican National Convention, Vice President Mike Pence laid out the consequences of Mr. Trump losing the election in stark, personal terms: “The hard truth is you will not be safe in Joe Biden’s America. Under President Trump, we will always stand with those who stand on the thin blue line.”

To defend Americans from the conspiracy to steal the election and destroy the country, Mr. Trump suggested that his supporters — the police, the military, bikers, construction workers — would confront his enemies in the streets, rhetorically deputizing his allies as a law unto themselves: “They’re peaceful people, and antifa and all — they’d better hope they stay that way.”