It’s hard to recall the last time a relatively short op-ed received the sort of instant vituperation rained upon Manhattan Institute fellow Heather Mac Donald’s recent piece in the Wall Street Journal. What explains it?

In her piece, Mac Donald parsed recent statistics showing an uptick in urban crime, which are not yet comprehensive but certainly suggest that the 20-year trend of reduced crime is being reversed. Sharp spikes in the homicide rates in Milwaukee and St. Louis, Atlanta and Chicago. The terrifying surge in Baltimore shootings since the riots last April mostly came too late for her overview.

Then she suggested a reason for the rise in crime: “The most plausible explanation of the current surge in lawlessness is the intense agitation against American police departments over the past nine months.”

The stats Mac Donald cites are what baseball fans would call a “small sample size,” and it might take several years of data to know conclusively that a crime upsurge is happening. But that can’t quite explain the strong language Mac Donald’s opponents deployed. “More sophistry than science,” opined Yale Law professor Tracey Mearnes. Bernard Harcourt, a Columbia professor, slammed Ms. Mac Donald for producing “fiction” designed to “undermine the recent gains of the country’s newest civil rights movement.” Liberal websites linked to these so-called rebuttals again and again, as if terrified that, if left unrefuted, Mac Donald’s arguments would gain irresistible momentum.

Why charges of “sophistry” and “fiction”—rather strong language from academics? The figures Mac Donald gathered were, everyone acknowledged, correct. Perhaps what was troubling was her quotations. One New York City had cop told her, “Any cop who uses his gun now has to worry about being indicted and losing his job and family.” A Milwaukee police chief is quoted as saying he has “never seen anything like” the current hostility towards police.

Or perhaps it was her suggestion that longstanding laws and due process procedures are in danger of being circumvented, to accommodate the new anti-police sensibility. In New York state, for instance, efforts are now underway to create a special prosecutor to put cops on trial when a grand jury doesn’t come forth with an indictment.

The overall result is what the St. Louis police chief called “the Ferguson effect”—cops are essentially retreating to a reactive mode, answering 911 calls and investigating crimes, but not going out of their way to proactively police crime-prone neighborhoods. Police officers simply don’t know how to deal with a dominant media narrative intent on portraying them as a violent blue gang.

But I suspect there is more to it than that. At a more emotive level, Mac Donald is being faulted for addressing honestly a difficult subject that most would prefer to avoid. The charge of pervasive police “racism” is, after all, driven in great part by the fact that police arrest, or stop and frisk, blacks at rates higher than they do whites or other ethnicities. But is racism the reason for this differential? In a Slate interview, Mac Donald notes that per capita rates of gun violence are 81 times higher in Brooklyn’s Brownsville, a predominantly black neighborhood, than in nearby Bay Ridge, which is predominantly white and Asian. Under such circumstances, should Bay Ridge be policed in exactly the same way as Brownsville? If you answer yes, you may establish your “non-racist” bona fides, but also that you couldn’t care less about the optimal allocation of scarce police resources in Brooklyn.

The difficult subject which Mac Donald gets to (more so in the Slate interview than in the Wall Street Journal piece) is that the underlying issue is not policing, but crime. People may not like the fact that blacks are arrested in disproportionate numbers, but this is due far more to disproportionate rates of criminal behavior than to racist police officers. It’s a fact that most people probably know and yet prefer not to state, which results in a stilted and not particularly honest national debate about policing.

Awareness of differential crime rates may be largely excluded from the debate, but so long as America is a democracy, it can’t be entirely squelched. Here we come to another reason for the strong reaction against Mac Donald: recent American political history. Rising crime helped the Republican party nationally in the Nixon and Reagan eras, helping to fuel several landslide victories. Republicans were not above subtle efforts to insinuate that Democrats were properly “the African-American party”—in some instances trying to spur demands that a black be placed on the Democratic national ticket.

Would such tactics work in today’s more multiethnic America? It can’t be ruled out. While there is little research on the subject, my impression from recent political battles in New York City is that Asian neighborhoods can be every bit as brittle in response to perceptions of black crime as the stereotypical white ethnic neighborhood of yore. Leading Hispanic politicians have spoken out against alleged police racism, but it may be more surprising how small a role Hispanic activists have played in the overall movement to depict cops as racist. Much of white America in the late 1960s approached racial issues imbued with an acute sense of America’s history of slavery and de jure segregation and more than a little bit of guilt—sentiments which new immigrants are likely to share little or not at all.

Concerns that broadening the discussion of “racist” cops to include rising crime rates threatens the national political fortunes of the Democrats are probably well-grounded. Writing as someone who supported Obama over McCain and Romney, and has no regrets over it, I think it is unlikely Obama could have been elected in a political atmosphere colored by urban rioting.

If their response to Heather Mac Donald is an indication, progressives feel the way to prevent Republicans from reaping any “backlash” gains is to place very tight boundaries around all discussion of cops, crime, and racism. It’s a strategy that is unlikely to work for very long in a genuinely free society.

Scott McConnell is a founding editor of The American Conservative.