Thanks to his bullying tactics, stressing the horror of a chemical attack and raising questions about America's credibility if it doesn't react to the alleged chemical attack in Syria no one in Congress any more dares publicly to raise questions about the lack of evidence. They would be painted by Obama's propaganda as not caring about US credibility and not caring about the victims of chemical attacks.
However, these Congress men still face the dilemma that attacking Syria will likely kill hundreds of Syrians and might well draw the US into a war. And they are perfectly aware both that the present lack of evidence would be insufficient to explain support for their decision - specially if things might go wrong. And they can also see that the same fluffy arguments could also be used to push them towards further involvement.