WASHINGTON -- President Barack Obama’s call Wednesday to thwart gun violence – with four children by his side – signaled the start of what will be an intensive campaign to sway public opinion on one of the most hotly contested issues in recent history.

Even before Obama walked off the stage at the Old Executive Office Building adjacent to the White House after calling on Americans to lobby their members of Congress to pass new gun laws, his advisers were cranking up the campaign machinery and tapping into social media. They unveiled a website, Now is the Time, and urged supporters to share it via Twitter, Facebook and email.

Obama will follow by taking his campaign on the road, as he and Vice President Joe Biden hit the trail in coming weeks to build public pressure on Congress to act on their proposals.

They’ll also engage key allies including Democratic governors who are championing gun restrictions, mayors of big cities and those who have experienced mass shootings in their communities. Several were invited to the rollout on Wednesday, including the mayor of Oak Creek, Wis., where a Sikh temple was the site of a mass shooting in August.

The National Rifle Association did not say how it will counter Obama’s campaign, but in an unsigned statement it criticized Obama’s approach as an overreach and said that it would work with Congress “on a bipartisan basis to find real solutions.” A fund-raising letter to the group’s 4 million members from its executive vice president, Wayne LaPierre, characterized the opposition to Obama’s plans as “the fight of the century.”

The White House believes public opinion can be mobilized and sought a broad array of groups to consult with, including law enforcement agencies and religious leaders. Biden said he’d talked to members of more than 200 groups in developing his recommendations.

“I have no illusions about we’re up against or how hard the task is in front of us,” he said.

Though solemn, the event in an auditorium at the Old Executive Office Building had the air of a campaign event: The room was packed with supporters who applauded as Obama delivered his remarks. As he left the room, Obama stopped to high-five the children who were placed on the stage behind him. White House aides said the children were there because they’d written letters to Obama.

Obama worked to delegitimize opposition, suggesting it would be driven by personal greed rather than any principle. He said pundits and special interest lobbyists would declare “a tyrannical all-out assault on liberty, not because that’s true, but because they want to gin up fear or higher ratings or revenue for themselves.”

Behind the scenes, he warned, “They’ll do everything they can to block any commonsense reform and make sure nothing changes whatsoever.”

He called for supporters to press their members of Congress and “get them on record,” asking whether they support universal background checks and renewing a ban on military-style assault weapons and high-capacity magazines.

The NRA delivered a pre-emptive strike late Tuesday, releasing a web video that called Obama an “elitist hypocrite” for not endorsing a proposal to install armed guards in all schools while his daughters are protected by armed Secret Service agents at their school.