In the final days before the election, Democrats in the closest Senate races across the South are turning to racially charged messages — invoking Trayvon Martin’s death, the unrest in Ferguson, Mo., and Jim Crow-era segregation — to jolt African-Americans into voting and stop a Republican takeover in Washington.

The images and words they are using are striking for how overtly they play on fears of intimidation and repression. And their source is surprising. The effort is being led by national Democrats and their state party organizations — not, in most instances, by the shadowy and often untraceable political action committees that typically employ such provocative messages.

In North Carolina, the “super PAC” started by Senator Harry Reid of Nevada, the majority leader, ran an ad on black radio that accused the Republican candidate, Thom Tillis, of leading an effort to pass the kind of gun law that “caused the shooting death of Trayvon Martin.”