Even among those who fought hardest to pass an ill-fated gun control bill earlier this year, the political reality was undeniable: If the Newtown, Conn., massacre could not persuade Congress to pass tougher laws, a mass shooting less than a mile away from Capitol Hill was not going to either.

Senator Joe Manchin III, Democrat of West Virginia, who proposed a bill that would have strengthened background checks, seemed at a loss when asked whether the shooting at the Washington Navy Yard had changed anything.

“We don’t know,” he told reporters on Tuesday. He suggested asking some of the members of Congress who had voted against his bill.

Senator Dianne Feinstein, Democrat of California, sponsored an assault weapons ban that managed to garner only 40 of the 60 votes needed.

“If I can find 20 people that want to change their mind, I’m ready to go,” she said, her tone acknowledging that it was extremely unlikely.

“I’m not optimistic right now,” she said. Then, asked whether the Senate majority leader, Harry Reid, should bring the background check bill back up for a vote, she responded, “I’m not going to tell him he should because I don’t want another loss.”

For his part, Mr. Reid agreed that gun control could not pass the Senate right now. “We don’t have the votes,” he said. “We don’t have them now.”

Even more modest measures to address gun violence, including ones with bipartisan support that would strengthen the nation’s mental health system, seemed unlikely to go anywhere.

Mr. Reid initially appeared to be open to the idea of advancing a mental health bill but then quickly backtracked. “I would be willing to do that, anything we can do to focus attention on these senseless killings that took place,” he said.

Later, his spokesman clarified that anything short of background checks was not enough. “It has to be in the bill,” the spokesman, Adam Jentleson, said.

During the gun control debate earlier this year, he had insisted that mental health legislation was not enough to address the problem and needed to be paired with a stronger background check system.

An amendment to the gun control bill that failed in April would have expanded federal financing for mental health programs. The amendment passed by 95 to 2.