Early signs say it will be hard for President Obama to win congressional authorization for military action in Syria. That could change; lawmakers might re-write the president’s draft authorization into something they can live with, ultimately allowing Obama to go forward. But whatever happens, Republicans have a compelling case for rejecting the president’s request. Based on off-the-record conversations with some of them, this is it:

1) The chemical weapons evidence. The Obama administration appears to believe that conclusive proof that the Assad regime used chemical weapons against Syrian civilians creates an unassailable case for U.S. intervention. A few lawmakers will likely challenge whether the proof is really conclusive. But a far larger number will accept the evidence that Assad used chemical weapons — and still reject intervention.