Here's what you need to know about the 'boyfriend loophole' holding up gun safety negotiations

Washington (CNN)"It's now or never."

That's how Sen. John Cornyn, the lead Republican negotiator on the bipartisan gun safety package, described last week's deliberations.

Talks to try to resolve key sticking points moved in a much more positive direction since Thursday, a source familiar told CNN's Lauren Fox on Sunday. But senators have been racing against the clock ahead of the July Fourth recess, which begins at week's end, to turn a general agreement on gun safety into legislative text that can actually clear both chambers.

The major sticking points? Funding for red flag laws and what to do about the "boyfriend loophole." Both issues present a number of thorny challenges for negotiators, but the "boyfriend loophole" specifically has been cited as a considerable roadblock.

Here's everything you need to know about the loophole, and why it's become such a challenge in gun safety negotiations.

What is the 'boyfriend loophole'?

The "boyfriend loophole" deals with whether unmarried partners can have guns if they were found guilty of violence against a dating partner.

Federal law currently only bars people from purchasing a firearm if they are convicted of domestic violence while:

Living with their partner

Married to their partner

Have a child with their partner

Democratic lawmakers have long sought to expand the law to extend that coverage to dating partners, convicted stalkers and any individual under a protective order. But as of now, it doesn't apply to other types of dating partners, hence the label "boyfriend loophole."

Closing the loophole would mean that anyone who was deemed to have been in a serious dating relationship and convicted of domestic violence would no longer be eligible to own a gun.

What does the research tell us?

Supporters of closing the boyfriend loophole have repeatedly pointed to data that they say underscores the need to address it.

Two separate but related studies worth reading:

An average of 70 women are killed by an intimate partner with a firearm every month, according to an analysis of US Centers for Disease Control and Prevention data by Everytown for Gun Safety.Access to a gun led to a notable increase in the risk of the woman being killed in a relationship with a history of domestic violence, according to a 2003 study funded by the National Institutes of Health.

More broadly, another study by Everytown for Gun Safety found that there is a direct correlation in states with weaker gun laws and higher rates of gun deaths, including homicides, suicides and accidental killings.

The non-profit organization's analysis put California at the top of the list for gun law strength -- with a composite score of 84.5 out of 100, with a low rate of 8.5 gun deaths per 100,000 residents, and below the national average of 13.6.

Hawaii has the lowest rate of gun deaths in the country with the second strongest gun law score. It also has the lowest rate of gun ownership, with firearms in 9% of households, the data shows.

Have any states closed the loophole?

At least 19 states have enacted laws that close the "boyfriend loophole," per data complied by Everytown for Gun Safety.

This includes: California, Connecticut, Delaware, Hawaii, Illinois, Indiana, Kansas, Maine, Maryland, Minnesota, Nebraska, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, Washington and West Virginia.

Other states -- like Alabama, Colorado, and Iowa -- prohibit convicted domestic abusers from having guns, but don't specifically close the loophole.

What's the hang up?

In their framework, lawmakers agreed to extend that prohibition on buying firearms to those who are convicted of domestic violence who are in a serious dating relationship, even if they aren't married or meet the other criteria.

But defining what kind of relationship would fit has been a point of debate within the group.

There has also been disagreement over whether there should be an opportunity for people who are convicted of misdemeanors to restore their ability to buy or own a gun down the line.

Cornyn said last week they were discussing whether a "misdemeanor conviction" for a partner could be enough to take away that individual's guns -- and how to define a domestic partner.

Describing the hangup, the Texas Republican said on Wednesday, the "issue has to do with the way that nontraditional relationships are handled in terms of domestic violence and misdemeanors. We've got to come up with a good definition of what that actually means."

He added: "What this does is it would add a category to, a bar for people being able to purchase a firearm if they fall in that category. So it's got to be clear and it's got to be something that can be actually applied because we're talking about pretty serious consequences associated with that."

Why do gun groups oppose closing it?

The National Rifle Association and other pro-firearm groups have long opposed closing the "boyfriend loophole," casting it as an attempt by Democrats to advance a gun control agenda.

Earlier this year, the NRA lobbied aggressively (and successfully) to keep any legislation addressing the loophole out of the reauthorization of the Violence Against Women Act.

Following news of the bipartisan gun safety agreement last week, the NRA said it will, "continue to oppose any effort to insert gun control policies, initiatives that override constitutional due process protections and efforts to deprive law-abiding citizens of their fundamental right to protect themselves and their loved ones into this or any other legislation."

How much sway that opposition will hold over what ends up in the final legislative text remains to be seen.

What else is in the gun framework?

Beyond seeking to address the "boyfriend loophole," the proposal announced last Sunday, "increases needed mental health resources, improves school safety and support for students, and helps ensure dangerous criminals and those who are adjudicated as mentally ill can't purchase weapons," according to a statement from the senators negotiating it.

The announcement included the support of 10 Republican senators, which would give the proposal enough support to overcome the Senate filibuster. But whether that support can be maintained throughout the legislative process will be a key factor to watch.