Over four days and three late nights of meetings, Democrats on the Senate Finance Committee have largely stood up to Republicans’ attacks on a proposal to overhaul the health care system. But behind the scenes and away from the C-Span cameras, their united front has given way to intraparty tensions, not just in the committee but in Congress generally. ... Senate Democrats have a filibuster-proof majority of 60 lawmakers. But a number of them are centrists, and the party cannot afford many defections, given Republicans’ nearly unanimous opposition. Further, seeking a semblance of bipartisanship, the White House still wants Ms....