WASHINGTON -- Liberals who see the effort to overhaul health care as a once-in-a-generation opportunity are growing anxious that a final deal -- and a Democratic president they backed -- will negotiate away their top priority: a public plan to compete with private insurers.

Some Democrats are threatening to oppose any bill that excludes this option, and sympathetic outside groups are pressuring wavering lawmakers. Health Care for America Now, a liberal group, and the American Federation of State, County and Municipal Employees...