President Obama addresses the nation from the East Room of the White House on Tuesday.

WASHINGTON – Vividly describing the deaths of more than a thousand people in an alleged nerve gas attack outside Damascus, President Obama spoke to the nation from the East Room of the White House to argue his case for why the United States must hold Syrian President Bashar Assad accountable.

Obama called the Aug. 21 attack a “crime against humanity” that violated international law.

“When dictators commit atrocities, they depend upon the world to look the other way until those horrifying pictures fade from memory,” Obama said. “But these things happened. The facts cannot be denied. The question now is what the United States of America and the international community is prepared to do about it.”

TRANSCRIPT: Obama makes his case to the nation

In his first prime-time address from the White House in more than two years, Obama found himself unexpectedly tasked with a dual challenge: bolstering public support for his decision to launch military strikes against the Assad government while explaining his decision to pursue a diplomatic alternative.

Obama called a new Russian proposal to have Syria turn over its chemical weapons stockpile “encouraging,” but cautioned, “It's too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime keeps its commitments.”

The almost contradictory messages reflected the unsettled state of affairs in the standoff with Syria over the alleged chemical attack. The White House and U.S. allies worked quickly Tuesday to explore the viability of the proposal made by Russian on Monday.

The Obama administration said it was skeptical that Assad's government would follow through with the plan. Still, in a meeting with senators on Capitol Hill, Obama asked lawmakers to give him time to sort through the options, and Senate leaders, in a sign of the deep reluctance to endorse the president’s push for another military intervention, readily complied.

But the president’s speech from the East Room was announced before Russia’s plan emerged. While diplomats in Paris, Damascus, Moscow and Washington worked through the details – running into early signs of the difficulty of crafting a workable plan – Obama continued his public relations campaign.

FULL COVERAGE: The debate over Syria

The speech, which aides rewrote on the fly Tuesday, was designed as the keynote of a week of meetings, briefings, speeches and phone calls aimed at lawmakers whose support Obama needs in his pursuit of congressional authorization for a punitive strike against Syria.

With polls showing Americans opposed to a strike by roughly a 2-1 margin, the White House sought to convince lawmakers to buck public opinion and back Obama on a vote some say could determine the future of his presidency and U.S. credibility abroad.

In his speech Tuesday, Obama stressed a strike would be limited – unlike the wars in Iraq and Afghanistan – but he answered some skeptical lawmakers who have wondered if a “pinprick” strike would be worth it.

“Let me make something clear: The United States military doesn't do pinpricks,” Obama said. “Even a limited strike will send a message to Assad that no other nation can deliver.”

But on Capitol Hill, resistance to military intervention seemed to firm up with the sudden appearance of the Russian alternative.

Senate Majority Leader Harry Reid (D-Nev.) indefinitely postponed an initial vote on an authorization resolution approved by the Foreign Relations Committee last week. He said he would be satisfied with a diplomatic solution. “I’m not a blood-and-thunder guy. I’m not for shock and awe,” he said.

Meanwhile, key senators began to draft language that would incorporate the Russian plan, perhaps authorizing force only if Syria refused to allow an outside entity, most likely the United Nations, to secure its chemical weapons stockpile. It could also require a U.N. resolution condemning Syria for using chemical weapons on its own people.

Sen. Carl Levin (D-Mich.), chairman of the Armed Services Committee, emphasized that the resolution was still being crafted. But he said the goal was to eliminate the threat of Syria using chemical weapons by keeping open the possibility of force, “like the Sword of Damocles over Assad.”

“It’s because of the threat of a strike by the president, because of the possibility that Congress would authorize it, that there’s movement at the U.N. So you’ve got to find a way to keep that pressure on. That’s the key to success at the U.N.,” Levin said.

The president told Democrats he needed a number of days to pursue the diplomatic channels. But he “was not overly optimistic,” said Sen. Richard Durbin (D-Ill.), the assistant majority leader.