No weapon has more clearly illustrated the debate over gun control than the semi-automatic assault rifle.

The weapon has been used in many mass shootings, yet is also one of the nation's most popular among gun owners with estimates of three to four million in private hands throughout the United States.

President Obama today urged Congress to pass comprehensive legislation that restricts military-style guns and ammunition, beefs up background checks and increases funds for mental health and school safety. Obama also signed 23 executive actions including more federal scientific research on gun violence and a modernized federal database system to track guns, criminals and the mentally ill.

PHOTOS: Where Gun Laws Are Most Lenient

"The type of assault weapon used in Aurora, (Colorado) is to pump out as many bullets as possible as quickly as possible, to do as much damage as possible," Obama said during a noon presentation at the White House. "That allowed the gunman in Aurora to shoot 70 people, killing 12. Weapons designed for the theater of war have no place in a movie theater."

A military-style assault rifle was also used in the Newtown, Conn., school shooting in December.

Surveys show that more than half of all Americans say the Newtown shooting incident have made them more supportive of gun control. At the same time, 58 percent support a ban on assault weapons, according to a Washington Post-ABC News poll released Monday.

While there is likely to be broad agreement in Congress to close gun show loopholes and tighten background checks, the idea of restricting the sale of assault weapons faces an uphill battle from gun rights advocates.

NEWS: How the Government Stifled Gun Research

"It's not so much the kind of weapon as who has it," said William Johnson, executive director of the National Association of Police Organizations (NAPO).

NAPO supported the initial assault weapons ban from 1994 until it expired in 2004, but has dropped its support in recent years.

"There has not been a demand from our rank and file that we need to do something about this," said Johnson, whose group represents various state and local police agencies on Capitol Hill.

The group also has opposed limiting the size of ammunition clips, something that is being proposed by President Obama, except armor-piercing or Teflon-coated bullets.

Some gun right advocates say that any proposed ban judges the weapon by its appearance rather than its capabilities.

"If you want to ban all semiautomatic firearms, you could talk about it," said David Kopel, research director at the Independence Institute in Denver. "But if you ban some semi-automatic weapons and not others then the ban is based on superficial accessories like a grip. That has nothing to do with public safety."

The administration's ban covers more than 100 specifically-named firearms as well as certain semi-automatic rifles, handguns and shotguns that can accept a detachable magazine, or rifles and handguns with a fixed magazine that can accept more than 10 rounds. Existing weapons are grandfathered, as well as 900 types of weapons used for hunting and sporting purposes.

Kopel says owners of the AR-15 and other semi-automatic rifles like it for sport shooting at a range, or to protect themselves from intruders.

NEWS: Advocates Looking to Fill Gun Control Loopholes

"The reason it's popular, is not because there are 4 million psychopaths across America, but they are popular for legitimate purposes," Kopel said.

He said the AR-15 is a frequent choice for police to carry in patrol cars. "It's often the best choice for lawful defense of self and others."

Kopel and others also say any ban violates Second Amendment constitutional guarantees to "keep and bear arms."

While the Supreme Court did strike down a handgun ban in the District of Columbia in 2008, it left open the question of what kind of weapons, and owners, could be restricted under state or federal law.

Another obstacle for gun reform is the culture of gun ownership in America, which often follows an urban-rural divide that parallels the split between politically red and blue states. President Obama noted Wednesday that his proposals need support from both sides of the country.

"The only way we can change is if the American people demand it," Obama said. "That doesn't just mean from certain parts of the country. We're going to need voices in those areas and Congressional districts where gun ownership is strong to speak up. It's not going to just be the usual suspects."