It’s all about small businesses. During a high-profile Supreme Court case about Internet sales tax on Tuesday, both sides emphasized the interest of small businesses in their arguments.

In South Dakota v. Wayfair Inc. (W) case, states want to reverse a 1992 ruling known as Quill v. North Dakota, which requires retailers to collect sales tax only if they have a physical presence in a state. They believe the ruling is obsolete, making states lose massive revenue while hurting small businesses.

“Our small businesses on Main Street are being harmed because of the unlevel playing field created by Quill, where out-of-state remote sellers are given a price advantage,” Marty Jackley, South Dakota’s attorney general, argued.

George Isaacson, a lawyer representing Wayfair and other online merchants in the case, believes small online businesses will bear the cost if the 1992 ruling gets overturned.

“If you increase the cost of admission, if you have barriers to entry, one of the inevitable effects is going to be that those small and medium-sized companies are going to be deterred and there will be even greater concentration by the largest retailers,” said Isaacson.

In a debate where opposite sides claim to protect small businesses, Justice Sonia Sotomayor asked, “Is there any brief I can read or any source to determine what constitutes a small business in America?”

It’s hard for small businesses to win

The case highlights how small and middle-size retailers have become the center of the debate in sales tax collections. Though President Donald Trump has repeatedly accused Amazon of not paying taxes, 19 of the 20 largest Internet retailers, including Amazon (AMZN) and Walmart (WMT), do collect taxes in most states since they have gradually established a local physical presence across the U.S. But smaller sellers have largely avoided collecting taxes because of ambiguous laws and complex tax codes that vary in each state. States argue this pits small businesses that are not headquartered in their hometowns against locally based retailers.

“Small businesses are not being treated fairly,” said Jackley. “We’re not asking remote sellers to do anything that we’re not already asking our small businesses to do in our state. And that is simply to collect and remit a tax.”

Justices also asked about the cost of implementing the sales tax program, and if there should be a minimum sales requirement on small businesses that would be subjected to the sales tax. South Dakota’s current law requires companies with at least $100,000 in sales or 200 transactions in the state to collect and remit sales taxes.