In the Democratic presidential debates, Hillary Clinton's sharpest criticism of her major opponent, Bernie Sanders, has focused on the immunity from civil lawsuits bestowed on the gun industry by Congress and President George W. Bush. Sanders voted for the immunity; Clinton voted against. However, the details and significance of the immunity - though so revealing of our paralysis on gun issues - have remained obscure.

The immunity was a response to innovative civil lawsuits brought against handgun manufacturers by more than 40 cities and counties and one state in the late 1990s and early 2000s. I conceived the city lawsuits while serving on a task force on youth and violence established by Mayor Ed Rendell, and was on some of the legal teams.

What led to so many lawsuits against one industry, and why would the gun manufacturers need an exemption from the usual rules?

Earlier litigation against gun manufacturers had been based on the design or defects of particular guns, and was rarely successful. The cities recognized that guns work all too well - taking about 75 lives each day, and creating a climate of fear that undermines the tenor and quality of city life.

The lawsuits focused on the central problem: mass murderers, criminals, and youths with easy access to guns. That access is provided, the cities argued, by the gun manufacturers' marketing and distribution practices.

Manufacturers regularly flood markets with their products. This is usually not a problem. But this product, unlike, say, iPads or refrigerators, is designed to kill. Manufacturers of far less dangerous or harmful products, such as spray paint and all-terrain vehicles, have imposed limits on distribution and sales.

But the gun manufacturers disdained even the most reasonable limits on purchases and purchasers, and pressed Congress and state legislatures to do the same. For example, they opposed the Brady Act record check, and still oppose its extension to cover all purchases, and they oppose any limit on the quantity of guns a person may purchase.

The manufacturers were particularly vulnerable to the marketing-based claim because of information they received from tracings of guns used in crimes regularly conducted by the federal Bureau of Alcohol, Tobacco and Firearms (ATF). The agency's attempts to identify a person who used a gun in a crime start with the manufacturer and serial number. ATF would call the manufacturer to get the name of the party to whom the gun was initially sold, then contact that person or company, and so on.

Each ATF trace places the manufacturer on notice both that one of its guns was used in a crime and which distribution channel, usually a distributor, led to a criminal use. There were about 200,000 traces a year. Since it turned out that more than half of the guns used in crimes were sold by less than 1 percent of the dealers, this information could have been used to limit or eliminate the supply of handguns to crime-prone distribution channels.

The primary city lawsuit claim was based on long-established law: The manufacturers' marketing and distribution practices created or contributed to a public nuisance, defined for hundreds of years as an unreasonable interference with public health or safety. Since the claim is solely focused on the marketing and distribution conduct of handgun manufacturers, it did not limit individual rights to buy or possess handguns. It steered clear of the Second Amendment.

Whistleblowers confirmed the obvious: The manufacturers knew their marketing practices made handguns easily available yet did nothing to alleviate the danger and harm. Smith & Wesson, the largest domestic manufacturer, settled, agreeing to change its marketing and distribution practices.

The manufacturers and the National Rifle Association went to Congress and state legislatures seeking protection from the city lawsuits in the form of broadly framed, retroactive legislative immunities. About half of the lawsuits were terminated by state legislative immunities. Of the remaining cases, about half got past the initial stages - doing better than the state tobacco lawsuits at comparable stages - but were terminated by Congress' immunity in 2005.

The result is that gun manufacturers, and only gun manufacturers, are not subject to otherwise available civil legal claims if they knowingly supply crime-prone distribution channels or engage in other conduct covered by the immunity that is usually civilly actionable. This was not tort reform legislation; no other industry, company, or individual is immune.

The manufacturers and the NRA also sought and got from Congress a statutory prohibition of use of crime-gun tracing evidence in civil actions, and restrictions on the information on crime-gun traces that ATF can provide to law enforcement, scholars, the public, or the manufacturers. They're the only manufacturers I've heard of who want to know less, rather than more, about their customers.

David Kairys, a law professor at Temple University, is the author of "Philadelphia Freedom: Memoir of a Civil Rights Lawyer," from which this is partially drawn.