If the Congress fails to approve authorization for a limited strike on Syria in response to the Syrian government's illegal use of chemical gas weapons to kill 1429 innocent civilians, including 426 children, I think there is zero chance that the Obama Administration would undertake future military action against militarily much stronger Iran for Iran's illegal development of nuclear weapons.
If Congress will not approve a military strike for an actual violation (in Syria), how could the Administration ever attack Iran for a theoretical violation for developing nuclear weapons?
I assume Israel and its supporters in Congress will be urging members of Congress to approve the President's request for authorization for a limited attack on Syria - because they understand how this Syria vote will impact any future options on Iran.