Democrats and physician advocates will call on Congress Wednesday to reverse a funding ban on gun violence research at the Centers for Disease Control and Prevention (CDC) that has been in place for nearly 20 years.

Right now, little is known about who is prohibited from owning guns, why people acquire them, why people commit gun violence and the amount of guns shared among young people. The exact number and types of guns in the U.S. are also unknown.

There's a reason for the lack of comprehensive data. Since 1996, Congress has placed an annual restriction on the Centers for Disease Control and Prevention (CDC). That year lawmakers slashed $2.6 million they had allotted to the agency to conduct gun studies. While the restriction hasn't explicitly banned the CDC from conducting research on gun violence, it has barred it from using federal funding to "advocate or promote gun control."

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That language, which emerged out of pressure from the National Rifle Association (NRA), effectively halted CDC's efforts to study gun violence because of concerns that it would risk losing even more funding.

Experts who study gun violence say without CDC research, many questions about gun violence remain unanswered.

"While we do have research showing the benefits of some laws such as handgun purchaser licensing with background checks for all handgun sales, there are relatively few studies of good scientific rigor that can tell us whether changes associated with gun laws are experienced by the individuals targeted by the law," Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research, told CBS News.

Most studies on gun control laws have simply reviewed whether rates of violence change after the laws have gone into effect, Webster said. But the effects of gun control laws haven't been analyzed at an individual level. Few studies have also been done on issues like non-fatal gunshot wounds or how people acquire guns illegally.

Conducting studies like that is both time-consuming and expensive, he added, and the lack of such data limits the scope of outside research on gun violence.

A month after the 2012 Newtown massacre that left 20 children and 6 adults dead, President Obama called on the CDC and other agencies to conduct and sponsor gun violence research.

But the original language imposed by Congress, as well as low funding levels have continued to have a chilling effect on the agency.

CDC health communications specialist Courtney Lenard told CBS the agency limited its research on gun violence not because it was legally prohibited, but because there always seemed to be a threat of additional funding cuts if the studies continued.

Technically, Webster said CDC could circumvent the restriction if it wanted to by addressing gun violence in the two areas that it already reviews: youth violence and domestic violence.

Even so, a group of House Democrats and advocacy groups for doctors will hold a press conference Wednesday morning on Capitol Hill calling for the language to be lifted from proposed GOP-sponsored spending bills that will likely be voted on next week.

Democrats have occasionally reinvigorated this debate since Newtown, and the pressure has been building this year.

After the Charleston shooting at a historic black church that left nine people dead in June, Democrats on the House Appropriations Committee offered an amendment to strip the CDC restriction from a bill that would fund the agency.

"When it comes to gun violence, my friends, this committee won't give one dime for the CDC to conduct research on something that is killing Americans by the thousands," Rep. Nita Lowey, D-New York, said during the markup of the bill.

Republicans on the panel, however, blocked the amendment because they claimed allowing the CDC to research gun violence would take aim at the Second Amendment. A GOP report accompanying the bill says the restriction is meant to prevent activity that would result in a future policy "to limit access to guns, ammunition, or to create a list of gun owners."

Several months later, 110 Democrats signed onto a letter in late October to then-Speaker John Boehner (R-Ohio) and House Minority Leader Nancy Pelosi, D-California, calling for the ban to be lifted.

"We are calling on you to take one simple step to help reduce the possibility of future tragedies like those in Roseburg, Charleston, Newtown, Aurora, and countless other American communities: lift the prohibition on federally funded research on gun violence in any final fiscal year 2016 appropriations legislation," the letter said.

Pelosi, who has called for a special committee on gun violence, was recently asked at a press conference why Democrats didn't remove the language when they held the majority between 2009 and 2011. She explained Democrats almost always faced GOP filibusters in the Senate.

Asked if the language could be removed now, Pelosi said, "I don't know if it's possible. We are not in the majority."

CBS News asked Speaker Ryan's office Tuesday about his current position on the ban and if he would be open to lifting it. A spokeswoman pointed to Ryan's comments from early November when he was asked about it at his first press conference as speaker.

"I'll refer you to the appropriators and Tom Cole, chairman of the subcommittee," Ryan told a reporter who had asked whether it is appropriate to maintain the funding ban.

On Tuesday morning, Ryan said GOP leaders would back a mental health reform bill after the shooting at a Planned Parenthood facility last week in Colorado Springs.

Tim Daly, director for campaigns, guns and crime, at the Center for American Progress, told CBS News a reversal on the CDC ban would require Republicans to change their minds or change their views on current restrictions.

"It is difficult to square elected officials on the one hand saying they want to end gun violence, and on the other hand blocking all avenues to understand the scope of the problem," Daly said.