On April 23, Suffolk University/USA Today released a poll of persons who say they are unlikely to vote in November 2018. Question 3 asks respondents if they are Republicans, Democrats, independents, or “something else.” The results: Democratic 27%, Republican 20%, independent 25%, “something else” 21%, refused 7%.
Another question finds that 57% of respondents favor a multi-party system. Only 22% said they were satisfied with a system that includes only the Republican and Democratic Parties. See this summary of the findings.