How 15 Democrats helped tank the 2013 assault weapons ban

— -- In the wake of the deadly shooting in Las Vegas, the debate over gun control is fiercely raging once again, with some hoping to revive a failed attempt to ban assault weapons in 2013.

That attempt came after the Sandy Hook Elementary School shooting in December 2012, when 20 children between the age of 6 and 7 years old and six school administrators were killed. The national tragedy prompted outrage and a swift call to action. Just a month after the shooting, Sen. Dianne Feinstein, D-Calif., proposed a bill that included a provision banning the sale, transfer and manufacturing of 150 specific weapons.

But the bill was defeated -- and Democrats were part of the reason it tanked.

Breaking down the vote

When the bill came to a vote on April 17, 2013, there were 40 votes in favor of the bill and 60 opposed to it.

In addition to Feinstein and the bill's 24 Democratic co-sponsors -- a quarter of the Senate -- 13 other Democrats voted for it. They were joined by Sens. Bernie Sanders, I-Vt., and Mark Kirk, R-Ill. On the opposing side, there were 44 Republicans, one independent and, somewhat shockingly, 15 Democrats.

While the 15 Democrats who essentially broke party lines and voted against the bill did have an impact on the optics of the vote total, they didn't entirely cause the bill's failure. Even if the 15 Democrats who voted against the bill had switched their votes, it would have been short five votes to pass because it needed 60 votes total.

What happened?

According to John Tures, a political science professor at LaGrange College in Georgia who has studied assault weapon bans, the slim majority that the Democrats held at the time played a big role in the vote.

After the 2012 election, Democrats had 53 seats in the Senate, while Republicans had 45 seats and independents held two.

"We'll remember that the Democrats held a majority but it wasn't a strong majority so they were concerned about losing seats, especially about in red states," Tures told ABC News.

"It made them very nervous about voting for it," he said.

Of the 15 Democrats who voted against the assault weapon ban, many of them hailed from traditionally red states like Montana, Louisiana, Arkansas, Indiana, Alaska, North Carolina and both Dakotas. Others were in swing states like Virginia, Colorado and West Virginia who could face significant challenges.

"The individual members were worried about losing their seat, and I think that in general, that as a whole they were worried about losing members," Tures said.

Beyond that, while there was a majority of public support for a ban, that was narrow as well. According to an ABC News/Washington Post poll taken from April 11 to 13, 2013, 56 percent of respondents supported a nationwide ban on the sale of assault weapons.

More generally, 52 percent of respondents supported enacting laws to limit gun violence.

Could a ban on assault weapons pass now?

Public opinion has shifted over time, however.

In a June 22, 2017 Pew Research poll, nearly 80 percent or more of both gun owners and non-gun owners support background checks for private sales and at gun shows, barring people on the no-fly or watch lists from buying guns, and preventing mentally ill individuals from buying guns.

There is more of a gap between owners and non-owners when it comes to banning assault-style weapons, but even that is nearing the tipping point: 77 percent of non-gun owners surveyed supported a ban on assault-style weapons and 48 percent of gun owners did too.

The spread was similar when it comes to banning high-capacity magazines, with 74 percent of non-gun owners and 44 percent of gun owners in favor.

And apparently, the Senate is listening.

Feinstein said on Oct. 3, two days after the Las Vegas shooting, that she was "considering" reintroducing the assault weapon ban bill, and on Oct. 4, she introduced a bill "to close a loophole that allows semiautomatic weapons to be easily modified to fire at the rate of automatic weapons."