As Congress returns to begin the debate over reshaping the nation's $2.2 trillion healthcare system, left-leaning organizations and liberal House members are telling their Democratic allies: Don't cave on us. The early skirmishing is perhaps the clearest indication yet of the uphill battle President Obama faces in delivering on his promise to make affordable, high-quality care available to every American. Disputes over whether to create a new government-sponsored insurance program to compete with private companies shine a light on the intraparty fissures that may prove problematic.