What backs it up is what the Jewish laws dictated and what the bible says the duties of the priests were and the fact that they did not even start those duties until they were 30. Jesus did not start His ministry until He was 30, you could not be a Rabbi until then also. JS didn't look up any of that info, obviously.
It is you that has no backing for your claim. I gave you quite a lengthy post on what immersion to the Jews meant---it's called a mikvah mikveh)---and you can look it up, though I doubt you will. It is still in practice today, it means immersion and was used for ritual cleansing----as stated in Leviticus 15. This did not require the presence of a priest. It was by self immersion. The priest themselves used the lavers in the temple for ritual cleansing and could not enter the temple without it or they would die. Baptism is not practiced in the OT for the cleansing of sins! It was for ritual cleansing of the body and was done morning, evenings, sometimes daily and more often. It was not a once in a lifetime occurrence, but daily and even more frequently, if needed. It was used for cleansing of the proselyte into the Jewish religion. John was not baptizing for the cleansing of sins either---that was done by the blood of animals still until after the death of Jesus, then it became a symbol for cleansing from sins. But it is still the blood of Jesus that cleanses from sin not baptism. Immersion then became a symbol to represent the death to sin and life unto Jesus.
Water does not wash away our sins---only blood can do that.
Heb_9:22 And almost all things are by the law purged with blood; and
without shedding of blood is no remission.
Luk 1:15 For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink; and he shall be filled with the Holy Ghost, even from his mother's womb.
Luk 1:16 And many of the children of Israel shall he turn to the Lord their God.
Luk 1:17 And he shall go before him in the spirit and power of Elias,
to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord.
Just a few of the sites:
What Is A Mikveh? | My Jewish Learning
Mikveh - Wikipedia
Mikveh