We begin with that new evidence on the type of poison gas used to kill in syria, and a clearer portrait tonight of that moment, the president surprised his own team by putting any action against syria... See More
We begin with that new evidence on the type of poison gas used to kill in syria, and a clearer portrait tonight of that moment, the president surprised his own team by putting any action against syria on hold, asking instead for a vote from congress. Right here on abc's "this week," secretary of state john kerry now saying lab tests have shown conclusive proof that it was sarin gas used in that attack in damascus. And tonight, we're learning more about the president's bombshell decision, to put military action on hold, surprising even his closest national security ed advised a vi advisers. The president now trying to convince members of congress, on the phone there with the speaker of the house. Tonight, we have our team on all of this. We begin here with our chief white house correspondent jon karl. Jon, good evening. Reporter: Good evening, david. Stunning is the only way to describe events this weekend. But now, the new white house mission is to convince a skeptical nation and a skeptical congress that it's time to act. Trying to convince the public and congress to support military action, secretary of state kerry went on five talk shows, presenting dramatic new details on the evidence poison gas was used in syria. From first responders in east damascus, we have signatures of sarin in their hair and blood samples. Reporter: Kerry told george stephanopoulos he's confident congress will approve the president's plan for a limited military strike. We cannot allow assad to be able to gas people with impunity. And I believe the congress of the united states will understand that and do the right thing. there? Will the president act anyway? The president of the united states has the right to take this action. Doesn't have to go to congress. But if I hear you correctly, you're saying that the president is going to act no matter what. No, I said the president has the right to act. So, will he? George, we are not going to lose this vote. Reporter: But how did we get from this -- I have decided that the united states should take military action against syrian regime targets. Reporter: To this? I will seek authorization for the use of force from the american people's representatives in congress. Reporter: The move surprised even the president's top aides. They expected an order to attack would come over the weekend. But at 6:00 p.M. Friday, president obama took a 45-minute walk around the south lawn of the white house, with his chief of staff. Telling him, for the first time, that he had decided to go to congress. The photos tell the story. Informing aides in the oval office friday night. Then talking it over saturday with his national security team. Some who expressed concerns about going to congress first. The biggest concern expressed by several members of the president's team was that congress could severely weaken and embarrass the president by voting no. But as you heard from secretary kerry, the team now says they are convinced that congress will ultimately approve the resolution. Jon karl leading us off. Jon, thank you. As jon pointed out there, secretary kerry sounding very confident that they will get the votes they need for action in syria, getting congress on the president's side. But we wanted to know, what would happen if that vote were held right now? Here's abc's senior washington correspondent jeff zeleny tonight. Reporter: The skepticism runs deep in congress. The united states went to war in iraq based on the false claim. Reporter: A rare sunday briefing at the capitol. More than 100 lawmakers, some still dressed for summer vacation, opened the syria debate. Congress won't vote on the war resolution until next week, giving the white house time to build its argument against the syrian government. If the vote was taken today, several lawmakers told abc news they believe it would fail. If congress acts like the british parliament, I don't think we having a are gaited our responsibility. Reporter: The outcome depends on unusual alliances. Some of the president's democratic allies are opposed to military action, saying they don't dispute chemical weapons, but are weary of another war. Is there another way to hold assad accountable? Reporter: And some of the president's republican rivals support military strikes. But hope he goes even further. The consequences of the congress of the united states overriding a decision of the president of the united states of this magnitude are really very, very serious. JEFF, WE HEARD senator McCain there. We heard from republican senator mitch McConnell who said president obama's role as commander in chief is always strengthened when he enjoyed the support of congress. Reporter: He's happy that the president came for support. He didn't promise that support. That's what's happening this week. The white house is working very hard to get their side of the story out and their case made, david.
This transcript has been automatically generated and may not be 100% accurate.