By turns angry and anguished, President Barack Obama declared on Tuesday that his “biggest frustration” since taking office has been America’s failure to take even modest steps to curb mass shootings so frequent that they threaten to become routine.

“The country has to do some soul-searching about this,” Obama said gravely in a town-hall question-and-answer session led by Tumblr founder and CEO David Karp in the White House’s State Dining Room.

“This is becoming the norm. And we take it for granted in ways that, as a parent, are terrifying to me,” he said. “I am prepared to work with anybody, including responsible sportsmen and gun owners, to craft some solutions. But right now, it's not even possible to get even the mildest restrictions through Congress. And we should be ashamed of that.”

Obama, who did not stifle his profoundly emotional response to the December 2012 massacre at Sandy Hook Elementary School, lamented that politicians “terrified of the NRA” have blocked limited responses such as tougher background checks.

“I have been in Washington for a while now, and most things don't surprise me. The fact that 20 6-year-olds were gunned down in the most violent fashion possible and this town couldn't do anything about it was stunning to me,” he told a roomful of Tumblr users. (Yahoo acquired Tumblr on May 20, 2013.)

“I've got two and a half years left. My biggest frustration so far is the fact that this society has not been willing to take some basic steps to keep guns out of the hands of people who can do just unbelievable damage,” the president said. “We're the only developed country on Earth where this happens. And it happens now once a week. And it's a one-day story. There's no place else like this.”

Fresh from his 2012 re-election, Obama pushed hard for a modest package of measures to curb gun violence in the aftermath of the Sandy Hook killings. But that effort failed in the face of a vigorous NRA lobbying effort that rallied bipartisan opposition.

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Rather than target his anger at Congress, Obama on Tuesday pleaded with Americans.

“The only thing that's going to change is public opinion. If public opinion does not demand change in Congress, it will not change,” he admonished. “I've initiated over 20 executive actions to try to tighten up some of the rules and the laws, but the bottom line is ... we don't have enough tools right now to really make as big of a dent as we need to.”

Obama underlined that Australia had sharply tightened its gun laws and has not had a mass shooting in recent years. "We have a different tradition. We have a Second Amendment. We have historically respected gun rights. I respect gun rights," he insisted.

But America needs “a fundamental shift in public opinion” and for voters inclined to back tougher gun restrictions to be as passionate as those who champion gun rights, the president said.

“Until that's a view that people feel passionately about and are willing to go after folks who don't vote reflecting those values — until that happens, sadly, not much is going to change,” he said.

Obama also took issue with those who argue that the problem is less guns than mental illness.

“The United States does not have a monopoly on crazy people,” he said. “And yet we kill each other in these mass shootings at rates that are exponentially higher than any place else. Well, what's the difference? The difference is ... these guys can stack up a bunch of ammunition in their houses. And that's sort of par for the course.”