If it happens, Jim Capretta writes:

If Brown were to win, it would send shock waves through Democratic ranks unlike anything we have seen in recent years. Democratic infighting would intensify. Many more closed-door meetings would be held as members vented and fought over what to do. It would takes weeks, not days, for this process to play out. There would be no health-care bill before the president is forced to deliver a state of the union address.
In the end, the White House and Senate Democrats might be able to convince enough House members to go along with some version of Plan B to get the bill passed and the debate behind them. But they would not get there easily or quickly.