WASHINGTON — President Obama, his face set with rage, stood in the Rose Garden surrounded by the families of Newtown and former Representative Gabrielle Giffords and asked how a measure to expand background checks for gun buyers — one supported by an overwhelming majority of Americans and a bipartisan majority of the Senate — had slipped away.

“The American people are trying to figure out,” Mr. Obama said, “how can something have 90 percent support and yet not happen?”

The answer: The measure never really had a chance.

In the nearly 10 years since the expiration of the assault weapons ban, even modest gun safety legislation has proved impossible to advance on Capitol Hill, where the momentum has been in the other direction, with lawmakers pushing various expansions of gun rights. The 68 votes last week to allow the debate on gun legislation to proceed was a mirage, a temporary triumph granted by senators willing to allow shooting victims and their survivors the vote they sought with absolutely no intention of supporting the final legislation and crossing the gun lobby or constituents who see gun rights as a defining issue.

While the opening vote provided advocates a glimmer of hope, the Newtown shootings, the tearful pleas of the parents of killed children and an aggressive push by the president could not turn the tide. They were no match for the reason Democrats have avoided gun control fights for years: a combination of the political anxiety of vulnerable Democrats from conservative states, deep-seated Republican resistance and the enduring clout of the National Rifle Association.