Following an hour of debate in the Colorado Senate, Democrats approved the bill, 18-16, on a party-line vote.
Republicans questioned the fiscal impact on state revenues and called it an attack on traditional marriage.
"There is no reason that we would have to do this piece of legislation if there wasn't such a complete and utter attack on the institution of marriage in the state of Colorado and in this body," Senator Ted Harvey (R-Highlands Ranch) said.
Democrats reject that reasoning.
"If people think that marriage is so fragile it needs to be referenced a couple times in the state income tax code to be protected then it really is in trouble," Pat Steadman (D-Denver), the bill's sponsor, said. "This not an anti-marriage bill, this is a tax-simplification bill."
Steadman says the proposal will likely affect the tax returns of perhaps thousands of couples with minimal impact to the state's bottom line.
The bill would adjust state statutes to link a resident's formal filing status on both Colorado and Federal tax returns.
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