If ever there were a moment for President Obama to learn from history, it is now, in the wake of Friday's shootings at the elementary school at Newtown, Conn. The timely lesson for Obama, drawn from the experience of Lyndon B. Johnson -- the last president to aggressively fight for comprehensive gun control -- is this: Demand action on comprehensive gun control immediately from this Congress or lose the opportunity during your presidency.In the aftermath of the assassination of Robert F. Kennedy (just weeks after the fatal shooting of Martin Luther King Jr. and only a few years after President John F. Kennedy was shot), President Johnson pressed Congress to enact gun control legislation he had sent to Capitol Hill years earlier. LBJ ordered all of us on his staff -- and urged allies in Congress -- to act swiftly. "We have only two weeks, maybe only 10 days," he said, "before the gun lobby gets organized."He told Larry O'Brien and me, "We've got to beat the NRA (National Rifle Association) into the offices of members of Congress."For three years Johnson's bill had been locked in the Senate Judiciary Committee by a powerful army of gun lobbyists. But LBJ was always poised to grasp any opportunity to achieve his legislative objectives, even in the most horrendous circumstances. He had used the tragedy of King's assassination in 1968 to "at least get something for our nation" out of it, finally persuading the House to pass the fair housing bill he had sent it in 1966. Johnson saw in the tragedy of the assassination of Robert Kennedy in June of 1968 a chance to get his gun bill enacted.Johnson's bill would ban all mail-order and out-of-state sales of handguns, shotguns and rifles; prohibit the sale of guns to minors; and require national registration of all guns and licensing of all gun owners. There had been little hope of enactment. But the day after Kennedy died from gunshot wounds, LBJ renewed his pressure on the Senate Judiciary Committee to report his bill out and send it to the Senate floor for a vote.Maryland Sen. Joseph Tydings, a close friend of Kennedy, and a couple of Democratic colleagues wanted the committee to consider a bill they believed was stronger. LBJ thought the Tydings proposal had no chance of passage and feared that any delay to consider it would give the gun lobby time to kill his legislation.That's exactly what happened. The Tydings proposal never got beyond the discussion phase, and LBJ's comprehensive bill failed by a tie vote in the committee. With relentless pressure, he was able to get a bill by October with a prohibition against mail-order sales (LBJ called it "murder by mail order"), sale of guns to minors and importation of "$10 specials," or cheap guns that were pouring into the country. But Johnson's pet proposals to license all gun owners and register all guns were defeated.When he signed the legislation on Oct. 22, 1968, LBJ vented his anger with the gun lobby for killing licensing and registration:"The voices that blocked these safeguards were not the voices of an aroused nation. They were the voices of a powerful gun lobby, a gun lobby, that has prevailed for the moment in an election year. ... We have been through a great deal of anguish these last few months and these last few years -- too much anguish to forget so quickly. So now we must complete the task which this long-needed legislation begins."Obama's pained remarks in the wake of this latest school shooting echo Johnson's anguish a half-century ago. But Obama has a unique opportunity: a lame-duck session of Congress. If he learns from the lesson of LBJ -- two weeks to get action -- and takes advantage of the fact that many members can vote their conscience without fear of retribution by the gun lobby because they are not seeking reelection, this nation may "complete the task" of passing comprehensive gun controls. That's an opportunity that is worth grasping out of the unspeakable tragedy that occurred in Newtown, Conn.Joseph A. Califano Jr. was President Lyndon B. Johnson's top assistant for domestic policy. He was secretary of health, education, and welfare in the Carter administration and is founder and Chair Emeritus of the National Center on Addiction and Substance Abuse at Columbia University.