Robert said
I don’t personally know of any specific scholars who currently defend this theory,* but it seems to be based on Epiphanius, who in the 4th century spoke of a pre-Christian Jewish heretical vegetarian sect known as the Nasaraians (Νασαραῖοι, Nasaraioi). Note that Epiphanius distinguished between the pre-Christian Nasaraians and the later Nazoraians (Ναζωραῖοι, Nazoraioi), who were Jewish followers of Jesus and the law of Moses. This latter group was named after Jesus’ village of Nazareth.

While Epiphanius does not say that John the Baptist belonged to this pre-Christian group of Nasaraians, the Mandeans, who do revere John the Baptist, sometimes referred to themselves as N’atsor’ayy’a (נאצוראייא), meaning the observant ones. That said, I would not put much stock in this theory. The Mandeans were eclectic and it is not usually thought that they were directly descended from John’s original followers. It is possible that Epiphanius had heard of some Mandeans, but the 4th century is pretty late. While Epiphanius does not refer to ‘Mandeans’ specifically, his description of the Nasaraians is similar in some respects, and several of the groups he describes are closely related to each other, especially through the figure of Elxai. 

I have also seen it speculated that perhaps Jesus was taking something like a temporary Nazirite (נָּזִ֗יר, nazir, similar sounding but different root) vow at the last supper: “Truly I tell you, I will never again drink of the fruit of the vine until that day when I drink it new in the kingdom of God.” Epiphanius does say that John the Baptist and James (the older step-brother of Jesus), but not Jesus, were Nazirites (Ναζιραῖοι, Naziraioi).

*A little bit of digging found two 19th century scholars who proposed a Mandean form of this hypothesis: Theodor Nöldeke, Mandäische Grammatik, 1874, p xx; Wilhelm Brandt, Die mandäische Religion, 1889, p 140. The Theological Dictionary of the New Testament, Vol 4, p 876 n 4 lists several scholars from the 1920s who also seem to have accepted some form of this view (Zimmern, Reitzenstein, Bultmann, von Gall, Schaeder, Kraeling, Lidzbarski). A more general form of the theory (without the Mandean component but with the Nazirite connection) seems to have been defended by Kennard into the 1940s. You can see (but not download) a free copy of his brief response to WF Albright (J. Spencer Kennard Jr. Nazorean and Nazareth, JBL 66;1(1947), 79-81) ** you do not have permission to see this link **.  

Thank you! That covers most of what I have been able to find out about this issue as well.

I would just add something concerning scholarship about the Mandaeans.

In this area of scholarship it has generally been true, as you put it, that “it is not usually thought that they were directly descended from John’s original followers.” Right now, though, the tide seems to be turning. More and more scholars are coming around to the view that the forerunners of the Mandaeans really did come from roughly the time and place of John the Baptist, and so may well have been among his followers (even if we can’t get much information about John through them).

Various reasons for this change in view have been offered, among them the difficulty of explaining the Mandaeans’ knowledge of  and ardent interest in, issues to do with Jerusalem and its temple, on the supposition that they were merely a Mesopotamian group with no historical connection to that area and time.

Charles Haberl (Rutgers) and James McGrath (Butler) have translated the Mandaeans’ Book of John, and both are sympathetic to the idea that the Mandaeans have some genuine connection to John the Baptist. They have articles available on ** you do not have permission to see this link **, in case you or anyone else is interested.