There are plenty of people who have weighed in

[Ksguo]

Hello,this is my first post on the forum.
Speaking to the Economic Club of Southwestern Michigan，former President George Bush defended his administration's interrogation policy and said，‘"Nothing I am saying is meant to criticize my successor.There are plenty of people who have weighed in. Trust me, having seen it first-hand. I didn't like it when a former president criticized me, so therefore I am not going to criticize my successor. I wish him all the best."

I don't quite understand the meaning of ‘weigh in’ in this sentence，and，who has seen it first hand？Bush himself or some else？
Thank you very much

[Davy07]

to join in an argument or fight
_________________
Only the Paranoid Survive

[Ksguo]

Thank You，Davy！

[Jamie (K)]

Remember that boxers or wrestlers are weighed before every match to make sure they are in the correct weight class. We say that they "weigh in" before the match. So as Davy says, the idiom means to join an argument or a fight, usually to join a debate by giving one's opinion on an issue.