WASHINGTON —  “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.

At a moment when the national conversation about how best to stem the menace of guns in the wrong hands seemed to have shifted, it turned out that the political dynamic had not.

Republicans armed themselves with disputed talking points from the gun lobby about how a bill to expand background checks and outlaw a national gun registry was instead tantamount to a national gun registry. Turning the dispute from gun safety to gun rights, they took to the Senate floor to denounce the compromise, even arguing with its sponsors, Senator Joe Manchin III, Democrat of West Virginia, and Patrick Toomey, Republican of Pennsylvania, two National Rifle Association-blessed lawmakers who could not contain their umbrage. Mr. Obama on Wednesday accused the gun lobby and opposition lawmakers of willfully lying about the measure.

Yet unlike fiscal fights, in which there are clear partisan divides, just enough Democrats broke with their party to make a difference. While the measure enjoyed the support of a broad swath of Democrats, the four who voted against it were just enough to give Republicans the numbers for the bill’s demise, along with the political cover that it was a bipartisan decision.

“It’s dangerous to do any type of policy in an emotional moment,” said Senator Mark Begichof Alaska, a Democrat up for re-election next year who voted with three other Democrats and 41 Republicans against the compromise. “Because human emotions then drive the decision. Everyone’s all worked up. That’s not enough.”

In truth, the Democratic support for the compromise background check measure was slightly better than it might have been, with Senator Mary L. Landrieu, a Louisiana Democrat who faces re-election next year, joining in support of it along with some other red-state colleagues who were iffy.

But after the vote for the assault weapons ban cost Democrats seats in 1994, red-state Democrats have steered clear of gun safety measures, judging that the political fury of opponents would not be offset by support from those who favor tighter controls.

As for the N.R.A., while some saw the group’s leader, Wayne LaPierre, as meandering and on defense after the Connecticut school shootings in December, seasoned lawmakers heard something far more telling: the group, which once supported new background checks, would no longer abide them. As a result, before a single hearing, bill or speech on the Senate floor, the legislation was in grave trouble. Then the Gun Owners of Americachimed in, attacking Republican senators who showed any interest in compromise, arguing that a national gun registry would arise from the bill.

The Senate’s rapid dismissal of what just weeks ago seemed the most achievable goal — a measure to extend background checks to gun buyers not currently covered by the federal system — sent the question of how and if to regulate firearms back to the states, where new laws both to restrain and expand gun rights are now fermenting.