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:
They seized Stephen and brought him before the Sanhedrin.
(6:15 also shows they were in the Sanhedrin.)
And Paul's presence is shown in 7:58b:
Meanwhile, the witnesses laid their clothes at the feet of a young man named Saul.
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.
It is quite likely that Paul had been a member of the Sanhedrin. Acts 26:10 said that he cast his vote (usually a colored stone) to condemn Xian Jews to death by stoning. This formal vote by Paul in Jerusalem where Paul was living, would have been reserved to the Sanhedrin that met regularly in that city. Also, remember that Paul said in Acts 22:3 that he was educated at the feet of Gamaliel, (who was a member of the Sanhedrin and a grandson of Hillel the Elder [who helped develop the Mishnah and Talmud]). Because Paul had such high credentials, in Acts 23:12 ff., he faced opposition of at least 40 men in Jerusalem who had dedicated themselves to fast until they had killed him. These 40 were all willing to risk execution by the Roman authorities, since they now guarded Paul. Paul's conversion was viewed by these Jews as a betrayal of the highest order, what one would expect if at some point Paul had been a member of the Sanhedrin.