The theory that Jesus was a Pharisee himself

[Statilius the Epicurean]

Jesus would have been influenced by Pharisaic teaching as most everyone in 1st century Judaism, but I'm not sure that's enough to say he was a Pharisee himself. The defining characteristic of the Pharisees was the Oral Torah, which at least in the gospels Jesus has an ambivalent attitude towards.

One of the most solid facts we have of Jesus' ministry is that he was originally a follower of John the Baptist, who seems to have less in common with the Pharisees and more in common with the separatist Essenes sect and the Qumran community which prioritised ritual purification in water. And the earliest Christian texts we have like the letters of Paul and the Gospel of Mark are more focused on an imminent apocalypse and the coming kingdom of God that is quite different to ordinary Pharisaic concerns. That's not to discount adoption of the ethical teachings of the House of Hillel in Matthew, for example, but the thrust of Jesus' message must have been significantly more radical than the Pharisee movement.