President Barack Obama’s nomination of Gen. David Petraeus has put some Democrats in a politically awkward position. While Senate Democrats are expected to rally behind Petraeus to replace Gen. Stanley McChrystal as the top commander in Afghanistan, they must also ask tough questions about when the U.S. will withdraw troops from the war-torn country. Many on the left are impatient with the Afghanistan war, and some Democrats are nervous much of their base may not show up to vote in this year’s midterm elections. Meanwhile, Republicans who are looking to drive a wedge between Democratic leaders and the liberal base...