WASHINGTON — Frustrated by deep political resistance to new gun laws, President Barack Obama on Thursday tried to regain momentum in his drive to pass legislation next month with an impassioned plea to remember the horror of the school massacre in Newtown, Conn., in December.

Standing in front of mothers of gun victims invited to the White House, Obama scolded lawmakers for not embracing the most sweeping of his ideas and objected to the notion that the country has moved on three months after 20 children and six adults were shot to death at Sandy Hook Elementary School.

“Less than 100 days ago that happened, and the entire country was shocked and the entire country pledged we would do something about it and this time would be different,” Obama said, his voice rising with indignation. “Shame on us if we’ve forgotten. I haven’t forgotten those kids. Shame on us if we’ve forgotten.”

The president’s remarks came as his proposal to reinstate an assault weapon ban has faltered in the Senate and another proposal to expand criminal background checks appears in trouble as well. Another measure to increase penalties on straw purchasers has more support. Obama described the gun package as one of his top priorities when he unveiled it in January, but he has come under criticism from some allies for not doing more to pressure lawmakers into adopting it. The Senate is preparing to begin a floor debate on gun laws when lawmakers return the week of April 8.

He seemed particularly irked at the suggestion that he had waited too long to push through his measures and had frittered away the political support for gun control in the wake of the Newtown shootings. A CBS News poll this week showed that support for tougher gun laws has fallen to 47 percent, down from 57 percent immediately after the massacre.

“We need everybody to remember how we felt 100 days ago and make sure that what we said at that time wasn’t just a bunch of platitudes, that we meant it,” he said. To lawmakers, he said, “Don’t get squishy because time has passed and maybe it’s not on the news every single day.”