Sani's defection was confirmed by his close aide, Malam Suleiman Ahmed, Daily Trust reported.
Ahmed said his principal defected on Thursday, September 16, when he had a meeting with some PDP leaders in Kaduna.
What does Senator Sani stand to gain by joining the opposition party?
In 2015, the outspoken senator and social critic was elected to represent Kaduna Central at the Senate on the platform of the ruling All Progressives Congress (APC).
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Between 2015 and 2019, Senator Sani stood out among his peers, standing as a voice of conscience. When the situation demanded, he didn't even shy away from criticising his (former) party.
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Apart from the important bills he sponsored/co-sponsored, one of the highlights of Senator Sani's term in the Senate was his revelation that he and his colleagues receive N13.5 million per month.
The outspoken senator also revealed that there is little accountability about how the money is spent.
In a daring move, he called for the payments to be scrapped, saying they are a key reason why many choose to enter politics.
He told the BBC:
"I decided to bust it open. It was a moral issue.
"The National Assembly is one of the most non-transparent organs of government. It pricked my conscience and I decided to burst the bubble and open the National Assembly to public scrutiny."
"If the expenses payment system was ended then parliament would only be attractive to people who contribute ideas."
Though Senator Sani wanted to continue serving at the Red Chamber, his clash with the Kaduna state governor, Nasir El-Rufai, robbed him of the APC's senatorial ticket.
He opted for a lesser-known party, the PRP, where he contested but lost to his former party.
Since his defeat in 2019, Sani's political influence has significantly reduced, becoming mainly a "social media critic".
At some point, Governor El-Rufai even teased him, saying the former senator has become a blogger.
“We have kicked out Shehu Sani, who has now become a blogger and Uthman Hunkuyi who has now relocated to China with his family… they were sent out for not giving the people of Kaduna state the opportunity of getting the loan that would have helped in improving the lots of Kaduna people."
With his defection to the PDP, Senator Sani stands to gain at least five things.
Increased political participation and influence
The PDP is Nigeria's lead opposition party and the only party at the moment that can effectively tackle the APC's dominance.
By joining the PDP, Senator Sani is returning from social media criticism to increased political participation and very likely become one of the influential leaders of the party in Kaduna state.
Ahead of 2023, if Senator Sani is interested in returning to the Senate, he stands a better chance of getting the ticket on the platform of the opposition PDP.
Judging from his previous term at the National Assembly, the Kaduna-born politician will play significant role if he returns to the Red Chamber.
Rathern than return to the Senate, Senator Sani may even switch things up and opt for the position of the number one citizen in Kaduna state.
In 2023, Sani's political enemy, Governor El-Rufai, will not be on the ballot as he would have finished the two terms guaranteed by the nation's constitution.
Thus, the chances of the former lawmaker getting the PDP's governorship ticket and eventually emerging as the next Kaduna governor in 2023 cannot be ruled out.
Should he fail to get any of the two political seats listed above, Senator Sani could also be appointed a minister provided the PDP dislodges the ruling APC from the centre in the next general elections.