WASHINGTON, D.C. — The Obama Administration is gearing up for the biggest foreign policy vote since  the Iraq War.

Administration officials argued Sunday that new physical evidence shows the Syrian government used deadly sarin gas on civilians.

The president wants to punish Syria for the attack.

But he also wants Congressional support.

It's now up to Congress to back President Barack Obama's plan to use force against Syria over its of chemical weapons against its own people.

"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," said Senator John McCain.

Congress returns next Monday. 

The White House won't say what the president will do if lawmakers vote against military action in Syria.