“We need law and order,” Donald Trump said when asked during Monday night's presidential debate about how to improve race relations in this country. What he meant by that was that police should engage vigorously in tactics like “stop-and-frisk” searches in which officers pat down people on the street to look for drugs or weapons, typically on little pretext. That, he insisted, is the only effective way to keep people safe in the inner city. “When it comes to stop-and-frisk, you know, [it] takes guns away,” Mr. Trump said. “Well, I'm talking about taking guns away from gangs and people that use them. ... Stop-and-frisk had a tremendous impact on the safety of New York City. Tremendous beyond belief.”

Post-debate fact-checkers quickly cleared up a couple of inaccuracies in Mr. Trump's description of stop-and-frisk. Indeed, a federal judge ruled that the tactic as practiced in New York City was unconstitutional — though Mr. Trump was right that Mayor Bill DeBlasio declined to pursue an appeal. And Hillary Clinton was right: Violent crime in New York has continued to drop since Mr. DeBlasio ended stop-and-frisk.

But the big lie about stop-and-frisk is that it is effective either as a tactic for getting guns off the streets or as part of a strategy to make a city safer. Our experience in Baltimore makes clear just how inimical it is to good police work and just how corrosive it is in terms of the relationship between police and the community.

The use of stop-and-frisk in New York exploded under former Mayor Michael Bloomberg, rising from under 100,000 encounters in 2001, the year before he took office, to more than 685,000 in 2011. That year, 87 percent of those stopped were black or Hispanic, groups that make up only about half of the city's population. Crime did drop in New York during that period, but not as much as it did in other cities, like Los Angeles, that did not engage in the practice. The Center for Constitutional Rights, which brought the stop-and-frisk lawsuit, responded yesterday to Mr. Trump's claims and noted that police reported about 106,000 major felonies in New York in 2011. In 2015, the last year for which statistics are available, police conducted just 23,000 stops, yet major felonies were slightly down, to about 105,000. The city saw 515 murders in 2011 but just 356 in 2015.

Part of the reason is that stop-and-frisk was stunningly ineffective at getting guns off the street. Testimony in the stop-and-frisk lawsuit revealed that New York police confiscated nearly 6,000 guns by using the tactic over an eight-year period, but it required 4.4 million stops to achieve that result — meaning officers found guns in about one out of every 1,000 encounters.

That suggests police were expending a lot of effort on a tactic that yielded few tangible results. But Baltimore's experience suggests the effects of such a strategy can be much worse than wasted time. The Department of Justice's report on Baltimore's Police Department found that here, like in New York, African Americans were by far more likely to be stopped than whites, relative to their share of the population, and that the “hit rate” for drugs or weapons in searches after pedestrian stops in Baltimore was comparable to that in New York — though it was substantially higher when police stopped whites than when they stopped blacks.

The effect of that discrimination in Baltimore, according to the DOJ report, is a common belief “that there is racism in law enforcement, unnecessary force and verbal abuse, an ‘us versus them' attitude among police officers, a lack of positive interactions with the police, and strong feelings of recrimination, resentment, fear and mistrust among residents.” That alienation and mistrust between members of the community and the police plays directly into the difficulty officers and prosecutors have in securing the cooperation of witnesses, solving crimes and sending the guilty to jail.

Mr. Trump said during the debate that politicians' failure to support tactics like stop-and-frisk is “very unfair” to minorities living in inner cities, but the record shows precisely the opposite. Ms. Clinton had it right when she observed, “It's just a fact that if you're a young African-American man and you do the same thing as a young white man, you are more likely to be arrested, charged, convicted, and incarcerated.” The policies Mr. Trump espouses would make that disparity worse. How he believes that's the answer to improving America's race relations, we cannot fathom.