My take on the delay until after the midterm election ...

If the Russian and Syrian forces would be bombing Idlib, there will be a chance for a release of a chemical substance by whatever party on the ground putting blame on Assad. What's not needed in these weeks a retaliatory reaction by Western allies to bomb Syrian facilities creating a nationalist feeling in the US thereby backing the sitting president and the Republican party. A reverse of wagging the dog in past war scenarios.

'Sapere aude'