With prospects for new gun control laws fading on Capitol Hill, President Obama and his allies on Thursday tried to regain traction on the issue of gun violence, shaming a Congress that has resisted his proposals to stem mass shootings.

Roughly 100 days after the massacre at Sandy Hook Elementary School in Newtown, Conn., Obama surrounded himself with mothers, some of whom lost children in the massacre, and insisted that it wasn't too late for Congress to coalesce behind new gun restrictions.

"The entire country was shocked and the entire country pledged we would do something about it and this time would be different," Obama said. "Shame on us if we've forgotten. I haven't forgotten those kids."

The president's call for greater gun control came in response to the Dec. 14 killings at Sandy Hook. But as time passes and emotions cool, Obama is worried he will lose the sense of urgency he needs to push through legislation calling for universal background checks and a ban on assault weapons.

His fellow Democrats in the Senate have already effectively killed the assault weapons ban and a prohibition on high-capacity ammo magazines by yanking those provisions from a larger gun control bill before the chamber could even vote on them. The president said little about those gun restrictions, however, and focused instead on universal background checks for gun buyers -- a proposal that faces less opposition in the Senate.

"The notion that two months or three months after something as horrific as what happened in Newtown happens and we've moved on to other things?" Obama said. "That's not who we are. And I want to make sure every American is listening today."

Some analysts said the president's insistent tone on Thursday was a departure for the typically even-keeled Obama, but that it hardly guaranteed success for his agenda.

"It was easy to detect a sense from Obama of real disbelief, mixed with anger, that people in Washington have forgotten about Sandy Hook," said Kristin Goss, a political scientist at Duke University and author of "Disarmed: The Missing Movement for Gun Control in America." "But there are a lot of folks [on Capitol Hill] still trying to figure out the politics of all of this."

Republican Sens. Marco Rubio of Florida, Mike Lee of Utah, Rand Paul of Kentucky, and Ted Cruz of Texas vowed to oppose Obama and filibuster even the watered-down gun bill, which requires 60 votes to pass.

And the National Rifle Association, which blocked virtually every gun control measure sent to Congress in recent years, dismissed the president's call for action.

"The economy is weak. National debt is mounting. Too many Americans are out of work, but Obama campaigns against guns to please the fringe," the group said.

The president has found an ally, however, in New York City Mayor Michael Bloomberg, who formed Mayors Against Illegal Guns and is using $12 million of his own money for television ads, including a new spot released Thursday featuring Newtown parents.

Yet, some wonder whether it's too little, too late. A new CBS News poll shows that less than half the country supports more gun restrictions, after nearly six in 10 backed such measures right after the 20 children and six adults were killed at Sandy Hook.