In 2007, then-Senator Barack Obama told The Boston Globe the following:
"The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."
The situation in Syria is very troublesome but we have not seen any evidence that this situation presents "an actual or imminent threat to the nation". We ask that now-President Obama holds himself to his own threshold for military action. Do not authorize military action in Syria without explicit Congressional authorization.
Source: http://www.boston.com/news/politics/2008/specials/CandidateQA/ObamaQA/