According to constitutional lawyers, if Anwar indeed had a majority of members of parliaments (MP) supporting him, there are several possible scenario can happen.
He could table a motion of no-confidence in the prime minister in Parliament. If the PM loses the vote, he will have to inform the Yang Di Pertua Agong (YDPA) that he no longer commands the majority and the government must resign.
Constitutionally, YDPA could then invite Anwar to form a government if he feels Anwar can command a majority, after which Anwar could then face a confidence vote in the house to show that indeed he has a majority.
Abdullah has another option if the opposition parties can successfully mount a non-confidence motion against him.
He can opt to seek dissolution of Parliament and in a snap parliamentary general election seek a fresh mandate.
Article form news strait time, 17september 2008
If Anwar has enough MP to crossover, he could just call a gathering of all defectors with signed pledges, parade them before the press, and practically bring down the government as he did in 1994 in Sabah after the Parti Bersatu Sabah democratically won the elections. At that time, Anwar was deputy prime minister and was heading the election campaign for BN in Sabah