In 2004 Michiganders approved a constitutional marriage amendment defining marriage as between one man and one woman, 58%-41%.
Today, as reported by Democrat-leaning Public Policy Polling, even less support legalizing same-sex marriage: 33% (with 53% opposed).
When given the option for civil unions, there is even less support for redefining marriage (full cross-tabs here):
"Which of the following best describes your opinion on gay marriage: gay couples should be allowed to legally marry, or gay couples should be allowed to form civil unions but not legally marry, or there should be no legal recognition of a gay couple's relationship?"
Gay couples should be allowed to legally marry: 29%
Gay couples should be allowed to form civil unions but not marry: 33%
There should be no legal recognition of a gay couple's relationship: 35%
Not sure: 3%
The poll respondents self-identified as 40% Democrat, 30% Republican and 30% Independent/Other.
Democrats support SSM 48%-36%. Independents oppose SSM 50%-36% and Republicans overwhelmingly oppose SSM 78%-10%.