President Barack Obama pressed Congress on Thursday not to forget the heartbreak of the Newtown elementary school massacre and “get squishy” on tightened gun laws, though some lawmakers in his own Democratic Party remain a tough sell on an approaching Senate vote to expand purchasers’ background checks.

“Shame on us if we’ve forgotten,” Obama said at the White House, standing amid 21 mothers who have lost children to shootings. “I haven’t forgotten those kids.”

More than three months after 20 first-graders and six staffers were killed in Newtown, Conn., Obama urged the nation to pressure lawmakers to back what he called the best chance in more than a decade to tame firearms violence.

At the same time, gun control groups were staging a “Day to Demand Action” with more than 100 rallies and other events planned from Connecticut to California. This was on top of a $12 million TV ad campaign financed by New York City Mayor Michael Bloomberg that has been pressuring senators in 13 states to tighten background-check rules. Targeted by Bloomberg’s ads are 10 Republican senators, including Saxby Chambliss of Georgia.

But if political momentum was building after the December shootings, it has flagged as the Senate prepares to debate gun restrictions next month. Thanks to widespread Republican resistance and a wariness by moderate Democrats from Southern and Western states — including six who are facing re-election next year — a proposed assault weapons ban seems doomed, and efforts to broaden background checks and ban high capacity ammunition magazines are in question.

In one statement that typifies moderate Democrats’ caution, spokesman Kevin Hall said Virginia Sen. Mark Warner is “still holding conversations with Virginia stakeholders and sorting through issues on background checks” and proposals on assault weapons and magazines.

In stronger language this week, Democratic Sen. Heidi Heitkamp of North Dakota said: “I do not need someone from New York City to tell me how to handle crime in our state. I know that we can go after and prosecute criminals without the need to infringe upon the Second Amendment rights of law-abiding North Dakotans.”

Expanding federal background checks to private sales at gun shows and online is the gun-control effort’s centerpiece and was the focus of Obama’s remarks. The system, designed to block criminals and the mentally disturbed from getting firearms, currently applies only to transactions by licensed gun dealers.

The National Rifle Association opposes the expansion, citing a threat that it could bring federal registries of gun owners, which would be illegal.

Democratic sponsors need 60 votes to prevail — a daunting hurdle since the party has just 53 of the Senate’s 100 seats, plus two Democratic-leaning independents. In fact, six Democrats voted last week for the GOP-sponsored proposal to require the 60 votes.

Exactly how they can achieve that has yet to be demonstrated, with Obama’s turn Thursday as arm-twister-in-chief underscoring the political pressure that proponents feel is needed 104 days after the Newtown killings.

While not naming the NRA, he chided opponents for trying to “make all our progress collapse under the weight of fear and frustration, or their assumption is that people will just forget about it.”