WASHINGTON - President 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, 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 e-mail. Obama will follow by going on the road, as he and Vice President Joe Biden seek to build public pressure on Congress to act.

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.

The National Rifle Association criticized Obama's approach as an overreach and said that it would work with Congress "on a bipartisan basis to find real solutions." A fundraising 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 NRA did not say how it will counter Obama's campaign. But as legislators, local officials and advocates on both sides of the issue can attest, the NRA has a political arsenal that can outmatch even the most powerful interest groups -- chambers of commerce, medical associations and police federations.

Just last week, Illinois state House members declined to consider proposed limits on assault weapons amid a surge of negative phone calls and e-mails from NRA members and other gun owners. In Wisconsin last week, legislators wary of provoking the NRA backed off a proposed ban on loaded firearms in the public gallery overlooking the state assembly chamber. In suburban Los Angeles, Glendale City Council members anticipate a packed and emotional meeting next week as a result of NRA-issued "grass-roots alerts" protesting proposed limits on gun shows. And legislators in at least seven states are weighing whether to allow public school staff members to carry weapons to work, as the NRA has urged.

The group and its allies, confident in their traditional ability to block congressional action, consider state capitals the prime battlegrounds and are planning to shore up their influence in states where they may be vulnerable, such as Maryland and other Democratic-led states.

Democrats also hope to gain momentum from the states: In New York, Gov. Andrew Cuomo on Tuesday signed gun legislation, and in Maryland Gov. Martin O'Malley has proposed several restrictions.

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. "I have no illusions about what we're up against or how hard the task is in front of us," Biden said.

The Washington Post contributed to this report.