Wheeler, whose son Ben was just six years old when he was shot dead by gunman Adam Lanza, made an emotional plea for gun control as lawmakers prepare to debate some of the most ambitious weapons restrictions in years.
"In the four months since we lost our loved ones, thousands of other Americans have died at the end of a gun," said Wheeler, her voice cracking as she described the memory of her son.
"Please help us do something before our tragedy becomes your tragedy."
She spoke two days after the Senate agreed to debate a bipartisan bill that would expand background checks for weapons buyers and increase the penalties for criminal sales, a deal widely hailed as a breakthrough in the push for gun control.
Yet, as Wheeler pointed out, that vote was "only the start." Lawmakers "haven't yet passed any bills that will help keep guns out of the hands of dangerous people," she said, "and a lot of people are fighting to make sure they never do."
"We have to convince the Senate to come together and pass commonsense gun responsibility reforms that will make our communities safer and prevent more tragedies like the one we never thought would happen to us," Wheeler urged. "Now is the time to act."
That message was echoed on the White House website, which on Saturday carried the banner: "Now is the time to do something about gun violence." The unusual decision to invite a citizen to give the presidential address follows a week of almost non-stop campaigning by Obama and other gun control advocates, including several Newtown families.
Senators are expected to begin debating the proposed reforms next week. The last time Congress passed any major gun control legislation was in 1994.