While the Democratic presidential candidates on Tuesday were debating gun laws that shield manufacturers and retailers from civil liability, a lawsuit that perfectly encapsulates that issue was working its way through Montgomery County court.

Lynsay Fox, the widow of Plymouth Township Police Officer Bradley Fox, sued an East Norriton gun store in 2014 for selling the weapon later used to kill her husband.

The suit, filed in coordination with the Washington-based Brady Center to Prevent Gun Violence, alleges that the store ignored warning signs that the buyer was purchasing the weapon for someone else. The guns ended up in the hands of Andrew Thomas, a felon who killed Fox and himself while fleeing a hit-and-run crash.

A trial remains months, if not years, away, but a judge in May tentatively allowed the case to proceed. That was in contrast to cases in Colorado and elsewhere that have been thrown out due to a federal law shielding gun manufacturers and retailers from liability.

Vermont Sen. Bernie Sanders voted in favor of that law, the Protection of Lawful Commerce in Arms Act, in 2005 and defended his stance Tuesday at the presidential primary's first televised debate.

"Do I think that a gun shop in the state of Vermont that sells legally a gun to somebody, and that somebody goes out and does something crazy, that that gun shop owner should be held responsible? I don't," Sanders said.

The issue set the Democrats' most left-leaning candidate apart from his opponents.

Front-runner Hillary Rodham Clinton voted against the bill and said it "give[s] immunity to the only industry in America. Everybody else has to be accountable, but not the gun manufacturers."

Former Maryland Gov. Martin O'Malley called it part of "a rigged game" and referenced the parents of a mass-shooting victim who were barred from suing the online company that had sold 4,000 rounds of ammunition to Colorado theater gunman James Holmes.

"Not only did their case get thrown out of court, they were slapped with $200,000 in court fees because of the way that the NRA gets its way," said O'Malley.

Michael Gottlieb, a Norristown lawyer who is defending some of the parties in Fox's lawsuit, said the federal statute does not apply to businesses that break the law, such as failing to perform a background check or selling to minors. He said it was designed to protect business owners from expensive lawsuits.

"By the time you get it to a jury, it bankrupts these people. Luke Kelly is going to go bankrupt. He will end up on welfare before this is over," Gottlieb said, referring to the owner of the East Norriton store. "That's their goal, to bankrupt anyone who's ever looked at a gun."

In an op-ed for the Huffington Post last month, Lonnie and Sandy Phillips, the parents of Aurora, Colo., shooting victim Jessica Ghawi, called the shield law "abhorrent."

"A person with outwardly obvious mental issues . . . was able to easily access a 100-round magazine and 4,000 rounds of armor-piercing bullets online without a valid ID," they wrote. "Online sellers, knowing they are shielded by immunity laws, refuse to put into place even minimal safeguards."

The judge cited both federal and state immunity laws in ordering the Phillipses to pay their opponents' legal fees.

In Wisconsin, on the other hand, a judge allowed a case to proceed against a shop that sold guns later used to shoot two police officers. On Tuesday, the Milwaukee jury returned a verdict of nearly $6 million against the store, whose guns had reportedly been linked to more than 500 firearms found at crime scenes in recent years.

In the Fox case, the store formerly known as In Site Firearms sold six guns over a three-month period to Michael Henry, an alleged drug addict who purchased the same type of gun several times and always paid in cash.

Henry pleaded guilty to the straw purchases and is serving a minimum 20-year sentence.

State Rep. Mike Vereb (R., Montco), who co-sponsored a law in Bradley Fox's name imposing tougher sentences on straw purchasers, said he's not aware of any efforts to pass a shield law in Pennsylvania.

"Juries are smart people, too, and they can see if a store followed the law," he said. "My concern with that, with the shield, is will that make some stores lax in some aspects?"

Rep. Steve Santarsiero (D., Bucks) said he opposes the federal shield law, and if elected to Congress in 2016, "I would vote to repeal it."

At the state level, both Santarsiero and Vereb said they were more focused on preventing people from getting illegal guns in the first place, rather than punishing third parties after the fact.

Santarsiero has scheduled a rally Thursday in Norristown to re-introduce a bill aimed at closing a background check loophole for private sales of long guns.

The bill failed to make it out of the Judiciary Committee last session but with amended language and a new Republican co-sponsor, Santarsiero said, "I think it will have a chance to get us more support."

The new House Bill 1010 includes a clause that allows private sales of long guns at gun shows as long as a licensed dealer conducts a background check and bears witness to the private transaction.

jparks@philly.com

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This article contains information from the Associated Press.