Board approval and shareholder votes are needed, Musk said, adding that the poll is to "test the waters" for a deal.
Tesla did not immediately respond to a Reuters request for comment, while xAI could not be reached.
As of 0234 GMT, more than 322,000 users had participated in the poll, with 70.2% voting in favour of the investment. The poll is set to close in a day.
Musk founded xAI last year as a challenger to Microsoft-backed OpenAI and Alphabet's Google, which are among those leading the fierce race for generative AI dominance.
Earlier in the day, Tesla reported its lowest profit margin in more than five years as the U.S. electric vehicle maker cut prices to revive demand and increased spending on AI projects.
Tesla said it expects its hardware-related profit to be accompanied by the acceleration of AI, software and fleet-based profits.
Last month, Musk said Tesla will likely spend between $3 billion and $4 billion on its purchases of chipmaker Nvidia's hardware this year.
Of the $10 billion in artificial intelligence-related capital expenditure this year, around half would be internal spend, he said.
xAI, which was valued at $24 billion, had raised in May $6 billion in series B funding, backed by investors including Andreessen Horowitz and Sequoia Capital.
