Normally, Main Street merchants are at odds with giant, national companies like Wal-Mart, Target, Home Depot and other big-box retailers. But when it comes to the sales tax loophole enjoyed by online retailers like Amazon.com, big retailers and small-town merchants are in agreement.

The ability to avoid collecting sales tax permitted by a 1992 Supreme Court interpretation of federal law gives Amazon and other online-only merchants an unfair price advantage.

The debate over sales tax avoidance by online merchants has been going on for a decade, and the main argument has been about fairness. It’s simply not fair for a consumer to buy a product from Amazon without paying sales tax, while the same product purchased from a downtown or mall-based store would have sales tax added to the final price.

The fairness argument remains. But today there is another factor driving efforts to eliminate the sales tax loophole — massive state budget deficits.

Over the past decade, states have lost out on tens of billions of dollars of sales tax revenue. That financial hit has no doubt been a frustration to state leaders working on annual spending plans, even if it benefits consumers. But today’s severe deficits in all but a few states have lawmakers in many capitals looking at recapturing lost revenue, as well as approving painful cost-cutting measures.

A coalition of retailers, called the Alliance for Main Street Fairness, has been associated mostly with small businesses in its agenda regarding the sales tax loophole. Now, the big national retailers have thrown their weight behind the effort, believing that state budget shortfalls will make it easier to bring an end to the sales tax advantage enjoyed by Amazon and other online-only retailers like Overstock.com.

The Supreme Court ruling determined that only retailers with a physical presence in a state had to collect sales tax. That means Amazon collects sales tax in only five states. Those are the states in which it has regional distribution centers.

Some states have begun to chip away at the loophole. In Illinois, Gov. Pat Quinn recently signed a law that targeted the 9,000 in-state companies (many of them quite small) that are so-called affiliates of Amazon, directing customers to its website store. In response, Amazon said it would drop all connections with those 9,000 Illinois-based businesses.

Despite Amazon’s hardball tactics, the issue is spreading from state to state — and should be addressed by Congress as a national issue.

The depth of the problem is illustrated by California, where it’s estimated only 1 percent of online shoppers complete the tax form for making voluntary tax payments for “use tax” — in lieu of sales tax. A report by California’s legislative analyst suggests $1.1 billion a year is uncollected because of the sales tax loophole.

Nationally, the sales tax loophole is estimated to cost states $8.6 billion a year, according to the National Conference of State Legislatures.

Congress should fix this problem by passing a law making sales tax collection universal, regardless of whether the retailer is a real bricks-and-mortar or an Internet-only operation. States, for their part, should continue the work being done in recent years to unify and simplify sales tax rules that are now quite convoluted.

In the meantime, Pennsylvania should join other states in moving to collect sales tax on Internet purchases made by state residents. It would not be a new tax, just an effort to capture uncollected sales tax allowed by a loophole in the law. Closing the sales tax loophole would also create a more level playing field between stores on Main Street or at the mall and Internet retailers.