President Obama is being shouted down by the gun lobby. He and Vice President Joseph Biden Jr. have spent weeks crisscrossing the country, making a forceful case for a package of laws that would reduce gun violence. At every stop, including one on Wednesday in Denver, he has demanded that Congress require universal background checks, ban assault weapons and large ammunition magazines, and prohibit gun trafficking. He has invoked the bloodshed in Newtown, Conn., and the daily toll that adds up to 30,000 gun deaths a year.

“If there is just one step we can take to prevent more Americans from knowing the pain that some of the families who are here have known, don’t we have an obligation to try?” he asked in Denver. “Don’t we have an obligation to try?”

But the president has been unable to break through the blockade set up by one of the most powerful and relentless lobbies in Washington. The assault weapons battle has already been lost, and it is increasingly doubtful that there will be enough votes in the Senate to support the expansion of background checks, the centerpiece of Mr. Obama’s agenda. (Sixty votes will be required to break the filibuster promised by the most extreme Republican senators, Ted Cruz of Texas and Mike Lee of Utah.) Even the gun trafficking provision, which seemed the easiest to pass, is being torn apart by the National Rifle Association, which put forward a substitute version that would eviscerate the prohibition on straw purchases of guns.

The gun lobby is a combination of forces that includes manufacturers, hunters and hobbyists, political opportunists, and a fanatically active faction that believes guns are needed to fight off the conquest of freedom by the government. That faction is represented by the group Gun Owners of America, which has spent the months since Newtown doing tremendous damage, insisting that expanded background checks will lead to a gun registry that will assist a secret plan by the president to seize every firearm.