One of Mr. Obama’s major goals has been to stop the production of new supplies of nuclear material; at the last nuclear security summit meeting, in 2012, he said “we simply can’t go on accumulating huge amounts of the very material, like separated plutonium, that we’re trying to keep away from terrorists.” But Pakistan has blocked his effort to negotiate a treaty that would end the production of more material — called the Fissile Material Cutoff Treaty — and it is unclear whether the summit communiqué will contain language urging other countries to disgorge their plutonium stockpiles.

There have been other obstacles to Mr. Obama’s agenda.

He succeeded in negotiating a modest arms control treaty with Russia in 2010, but the rapidly deteriorating relationship with Russian President Vladimir V. Putin has all but ended hopes for further reductions in the arsenals of the two countries.

Nonetheless, the effort to secure dangerous nuclear materials in Russia and the former Soviet states has been one of the big successes of the post-cold-war era: Just last year Ukraine, then still under the control of the ousted president Victor Yanukovych, sent more than 500 pounds of weapons-grade uranium from a reactor back to Russia. Ukraine gave up its nuclear weapons — left over after the fall of the Soviet Union — two decades ago. Had the weapons and materials remained in Ukraine, the current standoff with Russia might have taken on far more dangerous dimensions.

But Mr. Obama’s agenda has also run into major troubles in the Senate. In 2009 and 2010 the White House promised to reintroduce the Comprehensive Test Ban Treaty, which was defeated in the Senate during the Clinton administration. It has never been put back in front of the Senate, for fear of a second rejection. Even seemingly noncontroversial legislation, including passage of two nuclear terrorism conventions that deal with the physical protection of materials, has been stuck.

Both administration officials and advocates of major nuclear reductions argue that Mr. Obama has focused a level of attention on securing stockpiles even if his arms reduction efforts have come up short.

“What President Obama has done is put it more on the front burner and accelerated the process,” said Sam Nunn, a former Democratic senator from Georgia who played a central role in creating the American-backed program to help dismantle nuclear weapons and clean up nuclear material around the world.

“Significant progress has been made — not enough,” said Mr. Nunn, the chairman of the Nuclear Threat Initiative, a research group that presses for deeper cuts.