He is "not yet 100% certain" he will vote for it. But it may be too late for him, or to stop the bill:
Ezra Klein lays out the Dems' strategy: Mass officials delay certification of Brown if he wins, and Senate and House pass final bill before he gets there. if Nelson balks, this strategy fails.
Alternative: House agrees to Senate bill as is. No more votes needed. It goes right to Obama to sign. Then the reconciliation process used to amend some features House liberals want changed (financial only) - with 51 votes needed in Senate.