These sentiments can shift. A moment like this — a time when people are moved by tragedy and gripped by urgency — is often fleeting. Mr. Obama and those in Congress who share his goals must not miss this window. Mr. Obama seems to sense this, promising specific proposals “without delay” and to advance them in his State of the Union address.

The Justice Department drew up a list of recommendations last year to reduce gun violence — proposals that the president could implement by executive order, without waiting for Congress — but the ideas were shelved until after the election. We hope that Vice President Biden, who has been put in charge of the administration’s effort, will immediately pull them off the shelf.

Mr. Obama reminded us anew that the tragedy in Newtown, Conn., was caused by an individual wielding a military-style weapon. These firearms were intended for battlefields, not for self-defense or hunting. We agree with the president that “we should be able to keep an irresponsible, law-breaking few from buying a weapon of war.” If swiftly turned into law of the land, that would be a start toward responding to the horror of last Friday’s killings.