Four years ago, with Democrats poised to win the presidency and make major gains in Congress, liberals were salivating at what looked like the best opportunity to advance their agenda since President Lyndon B. Johnson’s Great Society.

In March 2008, activists gathered in Washington, D.C., for a conference put on by the group Campaign for America’s Future and enthusiastically laid out liberal policies on issues such as health care and “green” energy. Four years later, much has changed.