The U.S. Senate is expected to vote on a long-debated Internet sales tax law Monday, paving the way for millions of consumers to start paying sales tax on online purchases.

The legislation would allow the 45 states (and the District of Columbia) that currently charge sales taxes to require large online retailers to collect tax on purchases made by their residents. The law would only apply to online sellers that have sales of at least $1 million in states where they don't have physical operations, like a store or a warehouse.

The bill has a good chance of becoming law. It already received broad support in the Senate during earlier procedural votes, and now must pass Senate muster a final time. After that, however, it will need to be approved by the Republican-controlled House. Proponents argue that the proposal would not create a new tax, but rather enforce the collection of taxes already charged at brick-and-mortar retailers. Some House Republicans may view that as a tax increase.

If the bill is enacted, academic studies estimate that more than $12 billion in additional sales taxes will be collected from online purchases each year.

Related: What an Internet sales tax would cost you

Under current law, online sellers are only required to collect tax in states where they have a physical presence. Otherwise, consumers who shop online and don't pay a sales tax at the time of purchase are supposed to pay the tax to their home state. But estimates are that only about 1% of buyers comply with those widely unenforced laws.

"We think this will help level the playing field," said Stephen Schatz, a spokesman for the National Retail Federation, one of the bill's largest supporters.

Close to 30% of online shoppers surveyed by advisory firm AlixPartners recently said they would shop more at brick-and-mortar retailers if the tax became reality. Nearly half, though, said that an Internet sales tax would have no effect on their online shopping habits, according to the survey of about 2,500 consumers.

Related: Internet Sales Tax: What you need to know

After years of opposition, Internet giant Amazon.com (AMZN) is also supporting the bill, in part because the company is already collecting sales tax in nine of the states where it has warehouses.

Many other online retailers remain opposed to the legislation, saying that the sales tax would hurt business and create an administrative nightmare because they would have to determine tax rates for different states and localities at checkout. Anti-tax group, Americans for Tax Reform, has also come out strongly against the legislation, which it says "can only be viewed as a tax increase."

Meanwhile, eBay (EBAY), which has loudly opposed the tax, is lobbying for a broader exemption for small businesses.

"The solution is simple," CEO John Donahoe said in a letter to eBay users. "If Congress passes online sales tax legislation, we believe small businesses with less than 50 employees or less than $10 million in annual out-of-state sales should be exempt from the burden of collecting sales taxes nationwide."