premise: Nigerian vice president Atiku Abubakar's new party, the Action Congress (AC), Sunday slammed President Olusegun Obasanjo's declaration that his former deputy's office was now vacant as "unconstititional".
  Obasanjo on Saturday declared vacant the office of his embattled deputy after he defected to the rival opposition AC party ahead of the April 2007 elections.
  Presidential spokesman Uba Sani told reporters in Abuja Saturday that the decision had been taken in response to Abubakar's defection early last week.
  The vice president was expelled from the ruling People's Democratic Party (PDP) on Friday after he defected to the AC and was chosen as its presidential candidate for the 2007 elections.
  In its reaction on Sunday, the AC described as "unconstitutional, null and void" the declaration that Abubakar's seat had become vacant because he dumped the ruling party.
   What does Abubakar's dumping of the ruling party refer to?
hypothesis: Abubakar's defection