President Obama on Wednesday announced the most sweeping gun control initiative in two decades, proposing a ban on assault weapons, universal background checks for gun buyers and a 10-round limit on gun magazines in the opening salvo of a clash that the nation's most powerful gun lobby labeled "the fight of the century."

The president announced his plan -- and signed off on 23 executive orders to stem gun violence -- flanked by Vice President Biden, who helped craft the package, and children who wrote to the president last month after 20 elementary school students were gunned down in Connecticut.

"What's more important?" the president challenged lawmakers. "Doing whatever it takes to get an A grade from the gun lobby that contributes to their campaign or giving parents some peace of mind when they drop their child off for first grade?"

With the families of those killed at Sandy Hook Elementary School and in other mass shootings looking on, the president said more than 900 Americans were gunned down since the massacre in Newtown, Conn.

"Every day we wait," he said, "the number will keep growing."

Questions persist about how effective the president's proposals would be in reducing gun violence. Senior administration officials could not predict if the proposals would prevent mass shootings like the ones in Newtown or Aurora, Colo., and could not provide a specific time frame for new laws to take effect. Moreover, the legislative recommendations would not apply to weapons already purchased.

There was a muted response to the president's proposals on Capitol Hill, indicating the lack of enthusiasm by some in Congress for additional gun control measures. But the Obama administration and the National Rifle Association wasted no time exchanging blows Wednesday.

"I warned you this day was coming, and now it's here," NRA Executive Vice President Wayne LaPierre wrote in a fundraising letter circulated at a gun show Wednesday. "It's not about protecting your children. It's not about stopping crime. It's about banning your guns."

A day earlier, the NRA posted an online video calling Obama an "elitist hypocrite" for opposing armed guards in every school even though his own children have armed security.

White House press secretary Jay Carney called the video "repugnant and cowardly."

Obama is using executive orders to enact a number of his proposals without congressional approval. Those initiatives include more research funding for the Centers for Disease Control and Prevention on gun violence, stricter penalties for those who lie on background checks and allowing schools to use more grant money to hire school resource officers.

The president also called for the appointment of a new director to the Bureau of Alcohol, Tobacco, Firearms and Explosives and $4 billion in funding to keep 15,000 police officers on the street.

The president's plan scarcely addresses violent images in popular culture, like video games, even though the White House highlighted its meetings with entertainment executives when crafting the package.

Much as he did on the campaign trail, Obama called on the public to pressure Congress to approve his proposals.

"I intend to use whatever weight this office holds to make them a reality," Obama said. "If there's even one thing that we can do to reduce this violence, if there's even one life that can be saved, then we have an obligation to try."