Michael Dobie Michael Dobie is a member of the Newsday

Michael Dobie is a member of the Newsday editorial board.

PHILADELPHIA – On the third day, they talked about guns.

There aren’t many issues that have so divided this country, that have been so much of a litmus test for someone seeking political office, that have derailed so many candidacies. But as the battle continues, it’s becoming increasingly clear that Democrats don’t worry so much anymore about losing voters over gun control.

It’s been obvious in the way President Barack Obama has grown more frustrated and angry as he’s had to lead the nation through one period of mourning after another following our endless series of senseless shooting.

It was obvious when Virginia Sen. Tim Kaine, a longtime gun control advocate, went right at the National Rifle Association in his first appearance as Hillary Clinton’s running mate last Saturday in Miami. And it was obvious in the way the Democrats made it a theme at their national convention Wednesday night.

This is now a signature issue, and the party is going to ride it through November, wherever it leads.

Kaine told the crowd in the Wells Fargo Center of the pain he felt after the massacre at Virginia Tech in 2007, and using that to close a loophole in the state’s background check system that let mentally ill people purchase firearms.

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His credibility comes from his battles with the NRA, which grades him at F. And his argument is a common-sense one: that right of any individual to bear arms is tempered by that person’s responsibility to society.

More people are listening. Polling says so.

More people than ever favor background checks for private or gun show purchases, including two-thirds of Republicans and nearly 90 percent of Democrats. And in the last seven years, the number of Americans who say that laws that limit gun ownership are an infringement on their Second Amendment rights has dropped from 55 percent to 43 percent.

Kaine’s remarks only put an exclamation point on a program of testimonials from survivors of gun violence and from relatives of its victims, testimony that was emotional and angry, sad and demanding, stirring and rousing.

Christine Leinonen hushed a noisy crowd with a poignant tribute to her son, Christopher, killed in last month’s nightclub attack in Orlando, and she asked where common sense was the day he died.

Erica Smegielski, the daughter of the Sandy Hook Elementary School principal killed in that horrific attack in 2012, said flatly, “I should not be here tonight. I should be home with my mother,” and lacerated “cowering politicians.”

And former Arizona Congresswoman Gabby Giffords, who was shot in the head and nearly killed in 2011, took the stage to huge applause, her gait still impaired but her smile bright. “We have important work ahead of us, work that will determine the future of our country. Are you ready?”

It was powerful stuff, drawing a clear line of demarcation between Clinton and Donald Trump, which was the point. And it was punctuated by a performance of “What the World Needs Now is Love” from a cast of several dozen Broadway all-stars led by Idina Menzel and Audra McDonald, a reprise of the song they recorded to benefit the Orlando victims.

You don’t hear that kind of thing every day. But the Democratic Party said Wednesday night you’ll be hearing a whole lot of it from now on.