Since then he has been dogged by accusations of misconduct and investigations since becoming Senate president, though none has led to convictions.
Asked about the raft of defections, Saraki said: "I must commend them for their courage because it took a lot for them to be able to do that in the face of this level of intimidation and harassment."
The Senate president and his supporters say he has been targeted by political opponents, most recently on Tuesday morning when he said his movements, and those of his deputy, were halted by police. Images of the alleged incident were circulated on social media.
Police denied taking that action.
"The force wishes to categorically state that there was no authorized deployment of police personnel to besiege the residence of the Senate president or his deputy as reported in the media," a police spokesman said.
He said the allegations would be investigated.
HARASSMENT ACCUSATIONS
Amid an increasingly febrile political backdrop, lawmakers in June issued a series of demands to Buhari including a call for an end to the "systematic harassment and humiliation by the executive of perceived political opponents".
The president's supporters have previously rejected the accusations leveled against the former military ruler.
"I am committed to listening to very legitimate grievances and engaging all those who are aggrieved that we can see through their grievances," Adams Oshiomhole, the APC's national chairman, told reporters in Abuja after Tuesday's defections.
However, he said it was best for those motivated solely by personal interest to "return to where they belong".
Buhari said in April that he would seek another term. His candidacy depends on party approval, though that is usually considered to be a formality for the incumbent.
Nigeria's main opposition parties earlier this month agreed to form an alliance to field a joint candidate to contest the election.
*Reuters