A loose coalition of anti-tax activists, direct marketers and financial industry trade groups has a tough task this week in overcoming bipartisan support for letting states impose sales taxes on out-of-state sellers.

Big-box retailers and state governments showed last month that they command wide backing for the levy. Now, opponents are emphasizing the potential flaws in the legislation as they try to peel away that support.

“This bill is bad for business and bad for jobs,” said Senator Max Baucus, a Montana Democrat and chairman of the Senate Finance Committee, who said the measure wasn’t ready to debate yet. “It is full of unintended consequences.”