PROVIDENCE, R.I. — Throughout this year’s three-day Netroots Nations conference — as progressive activists have cheered on Elizabeth Warren, held Occupy-style general assemblies and attended seminars on topics ranging from voter ID laws to “Love, Compassion and Outrageous Forms of Activism” — present just below the surface of the discussion has been an undercurrent of dissatisfaction with President Obama. A series of liberal favorites, culminating with former Obama environmental adviser Van Jones, took to the stage at the Rhode Island Convention Center to rally the Democratic base — and also to make clear that nearly four years after his election,...