The president, who once talked about eventually ridding the world of nuclear weapons, faces enormous obstacles to any further reductions, both in Moscow and in Washington. President Vladimir V. Putin of Russia has demanded further concessions on missile defense before entertaining deeper nuclear cuts, and Republicans in the Senate have made clear they would resist any treaty that went beyond the New Start pact ratified in 2010. Mr. Obama’s aides have said that they have no appetite for another treaty battle, and that they would try to follow the precedent of the first President George Bush, who arranged reciprocal but not binding cuts with the dying Soviet Union without a treaty.

But the president’s return to the issue of nuclear reductions, however tentative, suggests he wants to invest at least some of his remaining time in office to making enough progress that he can point to it as an important legacy. “It’s a way of signaling what he sees his agenda for the rest of his second term being,” said an administration official, who, like others, declined to be identified in advance of the speech.

Supporters of further disarmament said they hoped Mr. Obama would refocus attention on the goal after rarely mentioning it in recent years, while he was consumed by other issues. “The most important thing he could do is lay out the broad agenda for the next three and a half years,” said John Isaacs, executive director of the Council for a Livable World, an advocacy group.

In addition to further reductions, Mr. Isaacs said, there are several policy changes Mr. Obama could take that would move the country further away from cold war-style national security. He said the president could take nuclear weapons off high alert and change nuclear doctrine to say that the only purpose of such weapons would be as a deterrent.

But administration officials cautioned against expecting the Berlin speech to be a sequel to Mr. Obama’s 2009 address in Prague laying out his vision for a nuclear-free world. Instead, nuclear reductions will be just one element of a larger, thematic address, officials said, and the president will not get into much detail about specific policy changes.