Amid fluid Syria situation, new dynamic for Obama address

Updated 7:45 p.m. ET

WASHINGTON White House speechwriters had to shift into rewrite mode.

President Obama's address to the nation Tuesday night will not be the speech he originally intended to deliver just 48 hours earlier.

It takes on a new dynamic because of Russia's offer and Syria's acceptance of a plan to put its stockpiles of chemical weapons under international control for eventual destruction.

Even so, Mr. Obama will use his speech tonight at 9 p.m. ET to again make the case that Congress authorize U.S. military action against Syria for its "brazen" use of chemical weapons against civilians on Aug. 21.

But Mr. Obama will have to address the Russian plan - about which he said yesterday "is a potentially positive development."

A senior White House official tells CBS News that the president will discuss the diplomatic possibilities now facing the U.S. and whether a diplomatic deal is possible.

In an interview with "CBS Evening News" anchor Scott Pelley, one of six network interviews the president did yesterday, he said that if a verifiable agreement on Syria's chemical weapons was reached, it would resolve "my central concern in this whole episode."

And this morning, the White House put out word that Mr. Obama conferred today about the Russia plan with allies British Prime Minister David Cameron and French President Francois Hollande.

A White House official says the three leaders agreed to "explore seriously the viability of the Russian proposal," and that process was beginning today at the United Nations. It would include talk of "a potential U.N. Security Council resolution."

"That, of course, would be the ultimate way to degrade and deter Assad's arsenal and is the ideal way to take this weapon away from him," said Secretary of State John Kerry Tuesday morning in testimony to the House Armed Services Committee.

• White House to work with U.N. on Russian proposal for Syria
• Syria backs Russia's plan for chemical weapons handover

So the possibility of a diplomatic resolution of Syria's stockpiles of chemical weapons takes some of the urgency out of Mr. Obama's call for military action.

But White House Press Secretary Jay Carney says "I think we have to be skeptical." He says the Russia plan has only "the potential for a positive development."

"It's not something that you can simply take the Assad regime's word for it," said Carney.

He made clear the White House view, as did the president in his interviews, that Syrian President Bashar Assad is only agreeing to a plan to surrender Syria's chemical weapons because of the threat of a U.S. military strike.

For that reason, the president will argue that the U.S. must keep the pressure on Syria and Congress must authorize the use of military force.

Mr. Obama knows he faces strong opposition in Congress and among the American people for a strike on Syria.

A CBS News/New York Times poll out Monday showed 61 percent of those surveyed oppose military action against Syria, while 30 percent favor it.

Mr. Obama says he understands that after 12 years of war in Iraq and Afghanistan, the American people are both weary and wary of a new commitment to military action. But he feels the need to explain why he's not talking about going to war.

It is also his intention to make the case that U.S. national security is put at risk if a dictator like Assad can deploy chemical weapons with impunity.

The senior White House official told CBS News that this speech isn't about conveying memorable quotes but it's to thoroughly explain why Syria matters, why the threat of military strikes is important and that there will be pursuit of a diplomatic remedy for a limited time.

The president's speech tonight will be his ninth address to the nation as president and the fourth time he has given such remarks from the East Room.

It was in that venue on May 1, 2011 that he announced to the U.S. and the world that Osama bin Laden had been shot and killed by American forces in a raid on his Pakistan compound.

Mr. Obama also used East Room addresses to the nation to announce a timeline for the drawdown of U.S. forces from Afghanistan and to take his case on deficit reduction and raising the debt limit to the American people.

At this same point in office, President George W. Bush had given 15 addresses to the nation, none of them from the East Room.