As the country has moved toward marriage equality, fewer and fewer democrats and independents support the idea of civil unions. The Dish reports:
The changing political composition of civil union supporters shows that the center of gravity of this debate has shifted significantly. The civil union option has moved from being a middle way dominated by political moderates a decade ago to one that is, today, most attractive to political conservatives. And looking ahead, there is evidence that the civil union option may have a limited future, at least if younger Americans are any indication. When given a three-way choice, civil unions are the least popular option among Millennials (Americans born after 1980).
It makes sense, as progressives shift to marriage equality and conservatives grab onto civil unions in the hope that they can slow or stall the marriage equality wave.