California has some of the strongest gun control laws in the nation, including the kind of ban on assault weapons that President Obama is calling for nationwide.

Yet Syed Rizwan Farook and Tashfeen Malik were able to obtain two semiautomatic assault rifles rather easily, it appears, and to take advantage of loopholes in California law to make the weapons even more deadly. And when state lawmakers passed a sweeping measure in 2013 intended to eliminate the loopholes and restrict more kinds of weapons, it was vetoed by Gov. Jerry Brown, who said the benefits would not outweigh the cost to gun owners’ rights.

That experience with assault-rifle limits in one of the country’s most liberal states is just one indication of how difficult it is to strengthen gun laws or achieve any agreement on what measures would help reduce overall gun violence and the threat of mass shootings.

On Dec. 3, the day after officials say that Ms. Malik and Mr. Farook fatally shot 14 people in San Bernardino, Calif., the United States Senate voted down two weapons proposals from Democrats. One would have expanded federal background checks to include purchases arranged online or at gun shows, and one would have prevented people on the terrorism watch list from buying firearms.