The time to act on gun violence has come, former U.S. Rep. Gabrielle Giffords -- wounded in a 2011 shooting -- told lawmakers Wednesday, opening the first congressional hearing on the issue since the Connecticut school shootings in December.

"Violence is a big problem. Too many children are dying. Too many children. We must do something," Giffords said. "It will be hard, but the time is now."

Sen. Patrick Leahy, the committee's Democratic chairman from Vermont, used his opening statement to call for stronger background checks and a crackdown on so-called straw purchases, in which people who can pass background checks buy weapons for others.

However, Leahy avoided endorsing an expanded ban on the assault-style weapons called for by Obama and fellow Democratic Sen. Dianne Feinstein of California.

"Second Amendment rights are the foundation on which our discussion rests. They are not at risk," Leahy said. "But lives are at risk when responsible people fail to stand up for laws that will keep guns out of the hands of those who will use them to commit mass murder. I ask that we focus our discussion on additional statutory measures to better protect our children and all Americans."

Also scheduled to testify to the Senate Judiciary Committee were Giffords' husband, retired astronaut Mark Kelly, as well as the executive vice president of the National Rifle Association, Wayne LaPierre.

Feinstein is also a member of the Judiciary Committee, and she is expected to clash during Wednesday's hearing with LaPierre, a longtime adversary in the political battle over gun control.

The hearing comes a few weeks after President Barack Obama's legislative proposals aimed at curbing gun violence following the December shootings that killed 20 children and six adults at Sandy Hook Elementary School. The shooter, Adam Lanza, also killed his mother and himself.

Obama's proposals include a ban on popular semiautomatic rifles that mimic assault weapons, a limit of 10 rounds per magazine, and universal background checks for anyone buying a gun, whether at a store or in a private sale. Guns sold through private sales currently avoid background checks -- the so-called gun show loophole.

The NRA, which is the public face of the powerful gun lobby, opposes many government limits on gun ownership as a violation of the constitutional right to bear arms.

Gun control advocates such as Feinstein and Vice President Joe Biden counter that the constitutional right can be limited, for example by the existing ban on private citizens possessing grenade launchers and other military weaponry.

"We must get away from a mind-set that has owners of firearms worried that 'they are going to take our guns away,' " said an op-ed by former Secretary of State James Baker, a Republican, and Democratic Rep. John Dingell of Michigan published Tuesday in The New York Times. "The Second Amendment guarantees that won't happen. Our nation has regulated various kinds of arms throughout history, and done so without violating the Second Amendment. We have, for example, restricted ownership of fully automatic weapons and grenade launchers."

A focus of Wednesday's hearing will be whether hunters and gun enthusiasts need semiautomatic rifles with high-capacity ammunition magazines, like one used in the Newtown school shootings.

LaPierre will tell lawmakers that more gun control laws are not the solution, according to prepared testimony provided by the NRA.

"We need to enforce the thousands of gun laws that are currently on the books," he said in the prepared statement. "Prosecuting criminals who misuse firearms works. Unfortunately, we've seen a dramatic collapse in federal gun prosecutions in recent years."

Federal prosecutions for gun violence plunged by 35% in 2011, according to LaPierre.

"That means violent felons, gang members and the mentally ill who possess firearms are not being prosecuted," he said. "And that's unacceptable."

LaPierre also will tell lawmakers to focus on fixing the nation's mental health system.

"We need to look at the full range of mental health issues, from early detection and treatment, to civil commitment laws, to privacy laws that needlessly prevent mental health records from being included in the National Instant Criminal Background Check System," LaPierre said in the statement.

The NRA's membership has spiked by 500,000 people since the Newtown shooting, bringing its number to more than 4.5 million, the group said Wednesday.

In the meantime, Kelly and Giffords have launched Americans for Responsible Solutions to push for gun control.

Kelly said he has not yet endorsed any legislation, but he supports Feinstein's bill to revive the expired assault weapons ban.

"We are going to work to pass some reasonable gun violence legislation that addresses universal background checks, closing the gun show loophole and helping with mental health issues, and banning high-capacity magazines," Kelly said. "Both Gabby and I are of the opinion that semiautomatic assault weapons should be left for the military to use."

Although Kelly has never met the star witness at Wednesday's hearing, he said he believes there are some things he and LaPierre could agree on.

"The NRA does some really good things. They teach people about gun safety, how to handle a firearm -- a lot of what the NRA does is really positive," Kelly said.

He said that despite the Tucson, Arizona, shooting that wounded his wife and killed six others, he and Giffords still support the Second Amendment, which guarantees Americans the right to possess firearms. Kelly said that he was such a gun enthusiast, he used to go to the NRA practice range outside of Washington.

"But we really need to do something about the safety of our kids and our communities. It's gotten really out of hand," he said.

CNN's Faith Karimi and Arielle Hawkins contributed to this report.