The Republicans did not block Obama from taking action. I'm pretty sure the existing AUMF was considered sufficient by most everyone at that time.
Obama was very vague about what he wanted to do. "Pin prick, not a pin prick something bigger, but not really big etc". He didn't give Congress much to work with. Also, Obama was looking for political cover, which the Repubs were not going to give him. Obama could have acted if he wanted, but as usual dithered until it no longer mattered) (Putin jumped up and acted instead.)
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The President has a Constitutional obligation to protect and defend the USA and her interests and therefore does not need Congressional action in doing so. Is gassing Syrian civilians a threat to the USA or her interests? I'll let others argue about that.
Fern