A growing chorus of liberal Democrats is pushing President Obama to get Congress's explicit approval before launching missile strikes into Syria. The pressure from supporters on Capitol Hill is not only a headache for Obama, but also puts House Minority Leader Nancy Pelosi (D-Calif.) and other top Democrats in the difficult position of bucking a broad segment of their own caucus in order to protect an ally in the White House. Scores of Democrats had signed letters a year ago advocating against military strikes targeting the forces of Syrian President Bashar al-Assad without congressional backing. Thirteen months later the enemy...