At a White House event with children who wrote him letters after the Dec. 14 school shooting in Newtown, Conn., President Obama today said the nation cannot wait any longer to do what can be done to reduce gun violence.

He unveiled a series of executive actions and calls for legislation that is sure to spark sharp debate in Washington. The steps he's taking, as expected, include calling on Congress to pass legislation banning "military-style assault weapons" and expanding background checks for gun buyers. He also announced 23 executive actions — such as "legal barriers in health laws that prevent some states from making information available about those prohibited from having guns" — that don't require Congressional action.

The president's various measures, the White House estimates, would cost about $500 million to implement. The Washington Post calls them "the most expansive gun-control policies in generations." They would be "the most sweeping changes to gun laws in nearly two decades," says the Wall Street Journal.

"Gun rights advocates," as NPR's Scott Horsley says, "have already promised stiff resistance." Even before the White House event, the National Rifle Association was calling the president an "elitist hypocrite."

While Obama promised to "put everything I've got into this," he also said that "pundits and politicians" will fight his proposals and claim he's waging an "all-out assault on liberty."

"The only way we can [get] change is if the American people demand it," he said. Americans, added Obama, need to "stand up and say 'enough.' "

The president also said that he has no wish to infringe on Americans' Second Amendment rights, but is taking "common sense measures that have the support of the majority of American people."

Vice President Biden, who joined the president at the midday event, lead the task force that developed the plan. The effort was launched in the days after the mass shooting at Newtown's Sandy Hook Elementary School.

We live blogged as the announcement was made and afterward.

Update at 12:45 p.m. ET. Key Points Of The Plan:

-- Obama is calling on Congress to pass legislation to "reinstate and strengthen the ban on assault weapons" that was in place from 1994 to 2004.

-- He wants a law passed to prohibit the sale of ammunition magazines that hold more than 10 rounds.

-- The administration says "Congress should finish the job of protecting law enforcement and the public by banning the possession of armor-piercing ammunition by, and its transfer to, anyone other than the military and law enforcement."

-- The federal government is taking executive action "to provide incentives for schools to hire school resource officers."

-- "The Departments of Education, Justice, Health and Human Services, and Homeland Security will release — by May 2013 — a set of model, high-quality emergency management plans for schools, houses of worship, and institutions of higher education, along with best practices for developing these plans and training students and staff to follow them."

-- An increase in efforts to "reach 750,000 young people through programs to identify mental illness early and refer them to treatment. ... The administration is calling for a new initiative, Project AWARE (Advancing Wellness and Resilience in Education), to provide this training and set up systems to provide these referrals."

-- Obama is directing the Centers for Disease Control and Prevention and other federal scientific agencies "to conduct research into the causes and prevention of gun violence." Congress has in recent years barred those agencies from using federal funds to "advocate or promote gun control." The White House argues that "research on gun violence is not advocacy; it is critical public health research."

Later today, our colleague Liz Halloran will be reporting about what's likely to happen to the president's initiative — particularly the legislation he'd like to see.

Update at 12:25 p.m. ET. White House Statements:

-- Summaries of the 23 executive actions.

-- A "fact sheet" on the president's plan.

-- A broader look at the plan.

Update at 12:16 p.m. ET. "Let's Do The Right Thing":

Speaking of those killed in recent mass shootings and those who try to protect Americans from such harm, the president ends his remarks by saying "let's do the right thing ... for them and for this country that we love so much."

Then, he sits down to sign the 23 executive actions.

Update at 12:10 p.m. ET. American People Must Demand Change:

While he vows to "put everything I've got into this," Obama also says that "pundits and politicians" will fight his proposals and claim he's waging an "all-out assault on liberty."

"The only way we can [get] change is if the American people demand it," he says. Americans, adds Obama, need to "stand up and say 'enough.' "

Update at 12:07 p.m. ET. Universal Background Checks, Ban On Assault Weapons And 10-Round Limit On Magazines:

The president calls on Congress to pass legislation that resurrects the expired ban on sale of assault weapons, puts a 10-round limit on ammunition magazines and creates a system of "universal background checks" — closing the so-called gun show loophole. "You should at least have to show you are not a felon or somebody legally prohibited from buying" a gun, he says.

Update at 12:04 p.m. ET. The Executive Actions:

The president begins to talk of the 23 executive actions he's taking today. They include support for schools that want to hire "resource officers" and develop emergency plans. And he's telling the Centers for Disease Control and Prevention to study the causes of gun violence.

Update at 12:02 p.m. ET. "We Can't Put This Off Any Longer":

Saying that "every day we wait," more Americans will be killed in gun-related incidents, the president says "we can't put this off any longer."

Update at 11:54 a.m. ET. "20 Beautiful First Graders":

Biden begins with words about those killed at Sandy Hook Elementary School, and he thanks the family members who have come to the White House today for the announcement.

Those killed at the school were "20 beautiful first graders gunned down in a place that's supposed to be their second sanctuary," Biden says, and six adults who were trying to protect them.

The nation, he adds, has a "moral obligation" to do what can be done to prevent another such attack.

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