The legislation has been decades in the making, and is currently being considered by the House of Representatives, where it will likely face some opposition.
Nonetheless, many states are hoping it will pass, saying an Internet sales tax will allow them to bring in much-needed revenue.
The total estimates of how much money states can make by taxing purchases from outside their borders remains to be seen, and S&P said it "believes revenues would be unlikely to boost tax revenues significantly in the near term compared with the size of total state budgets."
S&P also said there will be no near-term credit implications.
But if the bill is delayed for several years and states do not pursue other revenue sources, there is a possibility state revenues will erode. That, in turn, would hurt their credit, the agency said.
(Reporting by Caryn Trokie, additional reporting by Lisa Lambert in Washington)