ABC News' Jim Avila and Mary Bruce Report:

President Obama today vowed to never forget the 20 children killed in the Sandy Hook Elementary School massacre as he made an emotional and poignant plea for Congress to take action against gun violence.

"The entire country was shocked, and the entire country pledged we would do something about it and that this time would be different," the president said as he stood in the East Room of the White House with 21 mothers working to combat gun violence in America.

"Shame on us if we've forgotten. I haven't forgotten those kids. Shame on us if we've forgotten," he said, channeling the anger and frustration he expressed in the days after the December shooting in Connecticut.

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Adopting a somber tone, the president told the audience, which included the parents of victims of the Newtown shooting, that "we've cried enough" and it's time for Congress to act on the proposals put forth by Senate Democrats.

"Tears aren't enough. Expressions of sympathy aren't enough. Speeches aren't enough," Obama said. "What we're proposing is not radical. It's not taking away anybody's gun rights. It's something that if we are serious, we will do. And now's the time to turn that heartbreak into something real."

The president demanded that Congress not get "squishy" because time has passed since the deadly shooting and rebuked the "powerful voices" that oppose pending gun-control measures, saying they are "interested in running up a clock" and preventing tougher from happening.

"They're doing everything they can to make all our progress collapse under the weight of fear and frustration. … Their assumption is that people will just forget about it," he said.

Outside Washington, Obama said, the majority of Americans back his proposals to curb gun violence, which include expanded background checks and improved safety at schools.

"Right now, 90 percent of Americans - 90 percent - support background checks that will keep criminals and people who have been found to be a danger to themselves or others from buying a gun.

"More than 80 percent of Republicans agree. More than 80 percent of gun owners agree. Think about that. How often do 90 percent of Americans agree on anything?" he said to laughter from the audience. "It never happens."

The president also urged a vote on a "measure that would keep weapons of war and high-capacity ammunition magazines that facilitate these mass killings off our streets." The controversial assault weapons ban will not, however, be part of the package that Senate Democrats will introduce next month.

Obama said the votes in the Senate represent the best chance in more than a decade to reduce gun violence and he urged Americans to "raise your voices and make yourselves unmistakably heard."

"We need everybody to remember how we felt a hundred days ago," the president said, "and make sure that what we said at that time wasn't just a bunch of platitudes, that we meant it."