The main assumption of Bush's new Iraq tactical plan seemed to be that Prime Minister Maliki's government is a separate entity from the political forces around him. A indepedent central government, in Bush's formulation, should be willing to put down death squads and other terrorists in the interest of the "Iraqi nation."
If it's true that Maliki now sees himself as the "George Washington" of Iraq, willing to alienate -- or even attack -- those Shiite forces (supported by Iran) that elevated him to power, then Bush's argument (that Iraqi forces supported by US forces will put down the militias) makes sense. It also assumes that Maliki has the confidence that al-Sadr (or his successor) won't wait out the US presence, and rise against a (disloyal) Maliki in a year or two, after the US forces have withdrawn.
If, however, you believe that Maliki's rise is through the 'grace' of al-Sadr and other radical Shiites, then what Bush is asking for is for Maliki to turn on his own 'base' and embrace the cause of a 'greater Iraq.' At a practical matter, this appears to be protecting Sunnis at the expense of Shiites. (The Kurds seem, for all intents and purposes, to have removed themselves from the civil war in the South.)
Asking Maliki to turn on al-Sadr and the Shiites is something like (albeit on a lesser scale) asking Bush himself to turn on Christian evangelicals.
What did Karl Rove always preach about tending to the 'base'?