WASHINGTON — President Barack Obama urged US lawmakers to remember the children killed by gun violence and not weaken in the face of a powerful gun lobby as he reached out to moderate members of his own party before a Senate vote on gun controls expected next month. His comments Thursday came as new details were revealed in the December school shooting that left 20 young children dead and brought gun safety back into the national spotlight. Obama, flanked by grim-faced mothers who have lost children to guns as recently as last month, said Washington must do something. “Shame on us if we've forgotten,” Obama said. “I haven't forgotten those kids.” He said the upcoming vote is the best chance in more than a decade to reduce gun violence. The gun control legislation faces an uncertain future, even though more than 80 percent of Americans in polling say they support expanded background checks in gun purchases, which appears to be the most popular of the package of gun measures that Obama proposed just a month after the December shooting. The young gunman in that shooting used a high-powered rifle legally purchased by his mother, whom he also shot to death. Warrants released Thursday show that on the day of the massacre, Adam Lanza also took two loaded handguns to the school. A fourth gun, a loaded 12-gauge Saiga shotgun, was found in the passenger compartment of his car, along with 70 shotgun rounds. Obama encouraged Americans, especially gun owners, to press lawmakers to “turn that heartbreak into something real.” Among the 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. In Los Angeles, US prosecutors rebuffed Thursday a reported offer by Colorado theater massacre suspect James Holmes to plead guilty in exchange for escaping the death penalty, legal documents showed. A prosecutors' filing cited by the Denver Post and other media was scathing about a defense motion Wednesday which suggested a guilty plea in return for a sentence of life behind bars with no parole option. “There is not — and has never been — an actual or unqualified ‘offer' to plead guilty,” prosecutors wrote, saying they are “extremely unlikely” to accept an offer unless they get more details from Holmes and his lawyers. They want “specific access to information that would allow them to fully assess the defendant and his alleged acts for purposes of determining a just outcome to this case,” the newspaper cited the filing as saying. The 25-year-old is due in court Monday for a hearing at which prosecutors are expected to say whether they will seek the death penalty for the mass shooting, which left 12 people dead in July. Obama, also on Thursday tasked a presidential commission to probe long lines at polling stations and other irregularities faced by voters in last year's election. Obama, making good on a promise issued in his victory speech last November, signed an executive order to charter the commission, which will deliver a report within six months of its first formal meeting. The nine-member panel will be chaired by Bob Bauer and Ben Ginsberg, who served as top lawyers for the presidential campaigns of Obama and his Republican opponent Mitt Romney last year. . — Agencies