Until now, Obama had done little to change America's gun culture. But just days before his second inauguration, he appears determined to champion gun control in his next term, which also will be dominated by debt and spending fights with Congress and a likely debate over immigration reform.
His plan calls on Congress to renew a prohibition on assault weapons sales that expired in 2004, require criminal background checks on all gun purchases, including closing a loophole for gun show sales, and pass a new federal gun trafficking law - long sought by big-city mayors to keep out-of-state guns off their streets.
He also announced 23 steps he intends to take immediately without congressional approval. These include improving the existing system for background checks, lifting the ban on federal research on gun violence, putting more counselors and "resource officers" in schools and better access to mental health services.
Obama signed three of the measures at the ceremony.
Obama, who has called the day of the Newtown massacre the worst of his presidency, looked down into the audience and addressed the parents of one of the Sandy Hook victims, Grace McDonald, 7, saying he had hung one of her paintings in his private study.
"Every time I look at that painting, I think about Grace, and I think about the life that she lived and the life that lay ahead of her, and most of all I think about how when it comes to protecting the most vulnerable among us, we must act now," he said.