The Republican tax overhaul effort is in for a marathon debate on the Senate floor at the end of this month, with dozens of doomed Democratic amendments. But the real action will be elsewhere, behind closed doors.

Two parallel and largely private negotiations will determine the content of the bill that's due for a full Senate vote as early as Nov. 30: One is aimed at getting about a half-dozen wavering GOP senators on board. The other will attempt to smooth the path for a final House-Senate compromise in December.

Neither will be easy. GOP leaders must write a bill that can pass under the Senate's strict budget rules while cobbling together 50 Senate votes without alienating the House GOP's coalition of conservatives and moderates from high-tax districts. They'll also have to avoid political land mines such as the divisive health-care debate that has riven Republicans for much of this year.