ABC News' Michael Falcone and Shushannah Walshe report:

Earlier this month, after President Obama struck a deal with Republicans to avert the fiscal cliff, a top official from a leading liberal organization accused the president of being "clueless about how to use leverage in a negotiation."

Adam Green, a co-founder of the Progressive Change Campaign Committee, lamented in an interview with The New York Times, "Republicans publicly admitted they lost the tax debate and would be forced to cave, yet the president just kept giving stuff away."

But on Wednesday, Green and the Progressive Change Campaign Committee, as well as a host of other like-minded groups that have not been shy about criticizing the president and challenging mainstream Democratic orthodoxy, were singing a far different tune.

Stephanie Taylor, another co-founder of the liberal advocacy organization, offered support for the new gun control proposals unveiled at the White House just more than a month after the shooting at Sandy Hook Elementary School in Newtown, Conn.

"We applaud the White House plan to think big and take bold action against gun killings," Taylor said in a statement. "The PCCC is all in for this fight, investing time and money in Republican and Democratic districts until Congress passes major gun legislation that includes an assault weapons ban."

The Progressive Change Campaign Committee is one of several groups that broke with the president during the recent fiscal cliff standoff, but are now standing ready to unleash their troops of grassroots activists as his administration pursues changes to the nation's firearm laws.

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MoveOn.org, which also ultimately did not back the fiscal compromise earlier this month, on Wednesday praised what the group called "the most sweeping proposal in decades to reduce gun violence and make our communities, schools and streets safer."

"MoveOn members in every state and every congressional district are ready to stand up to the NRA and demand that Congress take action to reduce gun violence," said Garlin Gilchrist II, the group's campaign director. "Elected officials in Washington can no longer afford to sit back and wait for the next tragedy and issue hollow statements of sympathy."

Tom McMahon, executive director of the left-leaning Americans United for Change, said it would be "tragedy on top of tragedy on top of outrage" for Congress not to take action on the president's proposals.

"The leadership and lobbyists behind the NRA clearly do not give a damn about our public safety or what their own membership think[s]," McMahon said. "Their dream of bringing back the 'Wild West' with everyone strapped with an assault rifle, including teachers in classrooms, shows us they have either lost their minds or their last shred of human decency. Those that traffic in fear and profit by putting our communities in greater danger should be marginalized in this debate once and for all."

The groups are hoping to counter a chorus of voices, including the National Rifle Association and a growing number of lawmakers and elected officials around the country, that are already digging in their heels in opposition to the measures proposed by the Obama administration.

"The only way we can change is if the American people demand it," Obama said as he unveiled the proposals on Wednesday. "We're going to need voices in those areas and those congressional districts where the tradition of gun ownership is strong to speak up and to say this is important. It can't just be the usual suspects."

And these groups say they aren't just paying lip service to the president's call, they are already starting to take action.

MoveOn officials said their members plan to meet on Thursday at more than 230 community events across the nation to discuss the new proposals and find ways to put pressure on Congress. And more than 52,000 Progressive Change Campaign Committee members have signed a petition urging lawmakers in Washington to pass an assault weapons ban.

The president has also already found allies among the ranks of city and state officials like New York Mayor Michael Bloomberg, who is the founder of Mayors Against Illegal Guns and called the White House proposal "a bold and comprehensive plan to tackle gun violence."

New York Gov. Andrew Cuomo, a Democrat who signed a new gun control bill into law in his state this week, applauded Obama, too.

"Our nation has experienced far too much bloodshed as a result of gun violence, and the time for action is now," Cuomo said. "I commend the president for his leadership and swift action to stem the tide of gun violence, and I pledge my full support to seeing these proposals put into law."

Still, Obama's plan could hit a brick wall on Capitol Hill, where Republicans and even some Democrats are expressing doubt that it can pass.

"Nothing the president is proposing would have stopped the massacre at Sandy Hook," Sen. Marco Rubio, R-Fla., said in a statement. "As a strong defender of the 2nd Amendment, I will oppose the president's attempts to undermine Americans' constitutional right to bear arms."