While polytheism was the dominant religion among Arabs during Muhammad's youth, it's important to recognize that it wasn't the only one. Among the tribes of the Arabian peninsula during this time there were those like the Banu Qurayza, who were Jews. Considering the similarities between Judaism and Islam religiously, we should look to the presence of these groups as a potential source, though several of these tribes would end up opposed to the Islamic community during Muhammad's lifetime. The Quran also indicates that a number of Jews converted to Islam personally during Muhammad's lifetime.
There was also the tradition of the hanif. The hanif are considered in Muslim tradition to be basically "proto" Muslims that didn't necessarily affiliate with Judaism or Christianity. A cousin of Khadijah, Muhammad's first wife, is considered to have been a hanif and to be one of the first witnesses to Islam during Muhammad's early prophesying. This cousin, Waraqa, was a Christian (according to Aisha), indicating that the definition of "hanif" is somewhat vague and likely that it was used by early Muslim sources to mean "acceptably monotheistic" before the rise of Islam. We should also understand the idea of the hanif as part of the Islamic idea of the correct, basic religion. Islam holds itself to be not a development out of Judaism and Christianity, but as the original practice by Adam and Abraham which has since been corrupted, hence why converts typically refer to themselves as "reverts": they have returned to the true faith. As such, hanif are a sort of Muslims-in-waiting in Islamic historiography, detecting the failings of all corrupted religions but preceding Muhammad's revelation.
We should also consider Muhammad's revelation with this in mind. Some sources identify Muhammad as a hanif himself. The story of his encounter with the angel Djibril (Gabriel in the Jewish and Christian traditions) isn't dissimilar from certain Christian mystical or ascetic experiences. The desert has long been a place associated with religious purity and closeness to God in the Christian and Jewish traditions. In the Christian Bible, Jesus goes into the desert to be tempted by Satan. A similar situation appears in the writings of Athanasius from his hagiography of St. Anthony, called the Father of All Monks, who was also tested in the desert. Moses comes out of the desert with the news of God's protection (perhaps the closest parallel to Muhammad); John the Baptist lived in the desert and practiced asceticism; the Essenes, a Jewish sect at the time of Jesus, fled the corruptions of Jerusalem and lived in the desert anticipating the deliverance of the people (or the end of the world). Muhammad may have been influenced by these traditions.
Finally, Dr. Juan Cole has suggested that there was a closeness between Arab groups and Rome during late antiquity and the early middle ages. During this time, quarreling between Rome and the Sasanian Persians was constant, and minority groups like Arabs and Jews might have to pick sides in order to get by in regions like Arabia that were stuck in the middle of the fighting. While we shouldn't overgeneralize and assume that all Arab groups sided one way or the other, Cole’s thought was that most Arab groups preferred Rome, which in the period before Muhammad's revelation was staunchly Christian. The Christian influence of Rome would also have played a role in Arabic alternatives to polytheism/idol worship.
More can be said if someone has anything else to add; in the meantime you may be interested in this answer by /u/frogbrooks:
There's also a section on the FAQ about The Arab world before Islam for even more info.