With the return of 1960s-level civil conflict, various people are comparing Trump's law-and-order appeal to Nixon's in 1968 and '72. They are similar, but different enough to not get our hopes up about this issue giving Trump as much popularity as it did to Nixon.
There are two separate cycles going on: 1) societal cohesion, where periods of collective violence alternate with periods of collective harmony; and 2) individual opportunistic crime, where we see rising and falling rates of homicide, rape, etc.
Peter Turchin has described the cycles in societal cohesion, which show a 50-year period between high points (and between low points). The last low point was circa 1970, when there were race riots, students descending on the Pentagon to blow it up, labor strikes, and so on and so forth. The previous low point was circa 1920, which also saw even worse race riots, labor wars, and violent anti-war activism. Before that, it was the Civil War and Reconstruction Era of circa 1870.
That suggests a new low point will be reached circa 2020 -- of course, it could happen a little sooner or a little later. But we're already seeing the outlines of it, with black racial grievances taking collective and violent form against the government, although no real youth / student movement, no anti-war movement, and no labor uprising. Leading up to the November election, there won't be nearly the same level of collective civil conflict that there was by November 1968, let alone '72.
As for the rate of violent crimes that are individual and opportunistic (rather than collective and ideological), it has been falling steadily since a peak in 1992. It looked to have plateaued in the 2000s, but has dropped continually through this decade. The same goes for property crimes (burglary, theft, etc.).
In 1968, by contrast, violent and property crime rates had been soaring for nearly a decade.
Nixon was therefore appealing not only to those who were sick with how close the society felt to civil war, but also to those who were sick of having to watch their back when they left the house to run errands, and coming home to find it burglarized.
In 2016, the rise of ethnic group violence from blacks lashing out at the white government is becoming more and more worrisome to normal voters. If more of the Islamic terrorists turn out to be American residents rather than 9/11 style invaders, then that too will provide another example of collective violence by aggrieved ethnic minorities.
Still, it hasn't spread to students and anti-war activists setting off bombs, or striking workers defending their picket lines with force against scabs. And people have never felt safer from crime -- they have almost no stories about robbery, burglary, let alone rape and murder, that are circulating through their social circles, compared to how commonplace it was during the last crime wave.
Trump's appeal to law-and-order themes will be real, especially when running against someone who is not only weak on this issue, but broadcasts and even brags about her weakness -- which normal people will interpret as only likely to worsen the situation, as the violent groups smell fear among political leaders and really go in for the kill.
But we shouldn't think that these themes will be as important for voters now as they were in 1968 and '72, when the situation was a lot worse for both individual and collective violence.