President Barack Obama has urged US lawmakers to remember the children gunned down in America and not get "squishy" in the face of powerful forces up against gun control legislation, as supporters struggle to win over moderate Democrats before a Senate vote expected next month.

Obama, flanked by grim-faced mothers who have lost their children to guns as early as last month, said on Thursday that Washington must do something after the tragic mass shooting in Connecticut three months ago.

He called out to the families of four children killed at Sandy Hook Elementary School sitting in his audience.

"Shame on us if we've forgotten," Obama said. "I haven't forgotten those kids."

Obama's event comes as gun control legislation faces an uncertain future, even though more than 80 percent of Americans in polling say they support expanded background checks.

Backed by a $12 million TV advertising campaign financed by New York Mayor Michael Bloomberg, gun control groups scheduled rallies around the country Thursday aimed at pressuring senators to back the effort.

Obama said the upcoming vote is the best chance in more than a decade to reduce gun violence. He encouraged Americans, especially gun owners, to press lawmakers home from a congressional spring break, to "turn that heartbreak into something real."

"Don't get squishy because time has passed and it's not on the news every single day," Obama said.

'Powerful opposition'

Among the powerful forces opposing gun control is the National Rifle Association, a gun advocacy group that can put pressure on senators seeking re-election should they vote for restrictions the NRA opposes.

"We have a politically savvy and a loyal voting bloc and the politicians know that," said Andrew Arulanandam, spokesman for the NRA, which claims nearly 5 million paying members.

The heart of the Senate gun bill will be expanded requirements for federal background checks for gun buyers, the remaining primary proposal pushed by Obama and many Democrats since 20 young children and six women were shot to death in December at an elementary school in Connecticut.

Senate Democratic leader Harry Reid has said there are not enough votes to approve a ban on assault weapons, while prospects are uncertain for a prohibition on large-capacity ammunition magazines.The gun bill also increases penalties for illegal gun sales and slightly boosts aid for school safety.

Currently, the background checks apply only to sales by the nation's roughly 55,000 federally licensed gun dealers. Not covered are private transactions like those at gun shows and online.

The Senate measure is still evolving as Democratic senators Chuck Schumer, Joe Manchin and Republican Mark Kirk, use Congress' two-week recess to negotiate for additional support in both parties.

Expanding background checks to include gun show sales got 84 percent support in an Associated Press-GfK poll earlier this year. Near-universal background checks have received similar or stronger support in other national polls.