I remember hearing someone make the claim that Paul had been a member of the Sanhedrin prior to his conversion to Christianity. Is there any evidence for this claim? If so, what is it?
Stephen was stoned by the Sanhedrin and Paul (then named Saul) was present. The full story is told from Acts 6:8 to 7:60 but the relevant verses are:
(6:15 also shows they were in the Sanhedrin.)
And Paul's presence is shown in 7:58b:
Final note: in Acts 23:6 Paul himself appears before the Sanhderin and calls himself a "Pharisee".
Paul says he cast his vote against the believers he brought back to Jerusalem for trial (Acts 26:10). Paul also had authority from the high priest to arrest believers and punish them himself (presumably during interrogation - see Acts 26:11). This would be strange unless he was already some kind of officer of the court. One wouldn't think, for example, volunteers from Jerusalem's regular citizens would be authorized by the Jerusalem authorities to perform their will. The sense seems to be Paul already had some authority.
Moreover, Stephen seems to have been a member of Paul's synagogue - the Synagogue of the Libertines (Acts 6:9) where he disputed with the members there about Jesus, and the text says he was very convincing in that no one was able to resist his wisdom (Acts 6:10). It seems that there was a conspiracy against him according to Acts 6:11, but who are they who organized the effort to put him in jeopardy with the court? Paul admits in his letter to the Romans that his great sin was covetousness (Romans 7:7-11), and before writing this he wrote to the Galatians that he excelled above those his equals (Galatians 1:13-14). If he was zealous for the traditions of the fathers but couldn't resist the wisdom of Stephen, could Paul have been covetous of Stephen's ability to defend himself well against those traditions? If so, could Paul have been the one who reported Stephen and instigated the conspiracy against him. All this seems to point to Paul as being an important young man in the Jews religion before his conversion.