The White House is making the hard sell for a strike on Syria—meeting with strategically important Republicans, trying to persuade skeptical Democrats and sending top members of the Cabinet to testify before Congress.
President Barack Obama and Vice President Joe Biden planned to huddle with congressional leaders on Tuesday morning in the White House Cabinet Room.
Later in the day, Secretary of State John Kerry, who twice last week made a forceful moral argument for an attack on Syria, and Defense Secretary Chuck Hagel were due in front of the Senate Foreign Relations Committee.
The hearing, which also will include Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, follows a long weekend during which the administration backed off what appeared to be an imminent strike and began making its case to Congress and the public.
The administration argues that Syrian leader Bashar Assad must be punished for firing chemical weapons into a suburb of the capital last month, killing more than 1,400 people, including more than 400 children.
(Read more: As Syria boils, Obama looks to Congress for next steps)
On Monday, Obama met with two key Republican senators, John McCain of Arizona and Lindsey Graham of South Carolina. McCain, after the meeting, called on Congress to pass a resolution approving force against Syria.
Both Republicans said the goal of military action should be to "degrade" the capabilities of the Assad regime and "upgrade" those of the Syrian opposition, which has fought the Assad government for more than two years into a civil war.
McCain and Graham also criticized Obama for failing to make a clear case for intervention.
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Also Monday, Kerry, Hagel and National Security Adviser Susan Rice were among the administration officials who spoke to 127 House Democrats by conference call to argue for a Syria strike.
Sources told NBC News that House Democratic leader Nancy Pelosi came to the administration's defense, telling colleagues that preventing the spread of weapons of mass destruction was an important piece of national security policy.
(Read more: Russia raises alarm over Israeli missile test)
Kerry told the call that Congress faces a "Munich moment," a reference to the 1938 agreement that ceded part of Czechoslovakia to Nazi Germany and that was judged by history to be an appeasement of Adolf Hitler.
Kerry will go before the House Foreign Affairs Committee on Wednesday.
The president planned to fly overnight Tuesday to Sweden to meet with Scandinavian leaders there. He arrives Thursday in Russia for a meeting of the Group of 20 world economic powers.
Among those countries are Russia and China, which both have veto power in the United Nations Security Council—and would presumably block any American attempt to secure U.N. support for a Syria strike.
Obama, preparing to argue his case before Congress and the American public, said Saturday,"We should have this debate, because the issues are too big for business as usual."
(Read more: Syria's chemical weapons aren't a simple target)
In other Syria developments Tuesday:
—Israel test-fired a missile over the Mediterranean Sea, adding to tension in the region. U.S. military officials told NBC News that no American ships took part in the test, and called the test fairly routine.
—The United Nations said that more than 2 million Syrians have poured into neighboring countries, about 5,000 per day. A U.N. commissioner said Syria had become "the great tragedy of this century."
—A spokesman for the Syrian opposition said that a Syrian forensic medicine expert has evidence of Assad's involvement in a chemical weapons attack in March and has defected to Turkey.
—By Erin McClam, Staff Writer, NBC News. Reuters contributed to this report.