Obama accused gun-rights groups of lying about legislation which, he noted, polls show 90 percent of Americans supported.
"Instead of supporting this compromise, the gun lobby and their allies willfully lied about this bill," Obama said. "They claimed this bill would create a federal gun registry. ... Unfortunately this pattern of untruths about this legislation served its purpose."
A visibly angry Obama also attacked the process by which the amendment was rejected, noting that a majority of senators voted for it. Obama called the fact that the amendments required 60 votes to pass a "distortion of Senate rules."
"A few minutes ago, a minority in the United States Senate said it wasn't worth it," Obama said. "They block common-sense gun reforms, even as these (Newtown) families watched from the gallery."
Obama noted that 90 percent of Senate Democrats voted for the bill, while 90 percent of Senate Republicans voted against it — pushing back on the idea that the lack of unity among Democrats killed the amendment.
But he also said those who voted against it — including Democrats — were simply looking out for their political careers rather than trying to do something to save Americans.
"There are no coherent arguments as to why we didn't do this; it came down to politics," Obama said. "They caved to the pressure. And they started looking for an excuse — any excuse — to vote no."
Introducing Obama was Mark Barden, the father of one of the children killed at Sandy Hook Elementary.
"We are not defeated, and we will not be defeated," Barden said. "Every day as more people are killed in this country because of gun violence, we will grow stronger.
"Our hearts are broken," he added."Our spirit is not."