A bloc of several dozen liberals who are among President Barack Obama's closest allies in Congress is now becoming an obstacle to congressional approval of his plan for airstrikes in Syria.

Mr. Obama's quest to press ahead with an attack on the government of Syrian President Bashar al-Assad has stirred strong pushback among some Republicans. But dovish Democrats appear equally uneasy, and their numbers threaten to deal Mr. Obama his most embarrassing foreign-policy setback.

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