CLEVELAND, Ohio — Supporters of a bipartisan proposal to collect billions in unpaid Internet sales tax remain optimistic, even though the Senate vote has been pushed to May 6.

"It’s not a matter of if it’s going to pass, it’s a matter of when," said Michael Kercheval, president and chief executive of the International Council of Shopping Centers, the global trade association for the shopping center industry.

The Marketplace Fairness Act is designed to enable bricks-and-mortar stores to better compete with online sellers like and Zappos that don’t charge sales tax, and therefore get a price break of 6 percent to 10 percent over local stores.

The measure would enable states to charge and collect an estimated $23.3 billion in taxes that now go unpaid, according to the National Conference of State Legislatures.

That includes an estimated $628.6 million that would have gone to the State of Ohio and local governments. Although consumers who buy from out-of-state sellers are supposed to keep track of their sales tax and pay them on their state income tax forms, less than 1 percent of taxpayers actually do.

That’s because a 1992 Supreme Court ruling, Quill Corp. v. North Dakota, exempts online retailers from having to charge sales tax in states where they doesn’t have a physical store or warehouse.

But supporters of the Marketplace Fairness Act say that 21-year-old decision was issued long before Internet retailing became such a force and no longer applies to today’s cut-throat retail scene. They point out that the challenge of computing sales tax in what was then 7,600 local taxing districts isn’t as much of a problem for today’s online software programs.

Critics of the bill have called it a tax hike, saying it will increase costs for smaller businesses who sell online. And some consumers who shop almost exclusively online object to having to pay more for things they now buy "tax-free."

Although earlier versions of e-fairness bills have died on the floor, proponents have insisted from the beginning that the 2013 version was destined to pass.

That’s because this one irons out some of the differences between previous House and Senate versions, by providing exemptions for small online-only businesses that raked in less than $1 million the previous calendar year., the world’s largest online store, supports the bill, says it would prefer a federal law that applies to everyone rather than having to comply with different rules in every state. But eBay Inc., opposes the proposal because it says the $1 million threshold is too low; it thinks the bill should exempt businesses with up to $30 million in annual sales.

Sen. Sherrod Brown, Democrat of Ohio, voted on Thursday with the majority, 63-30, to end debate on the bill. Sen. Rob Portman, Republican of Ohio, did not vote. The Senate is set to vote on the bill on May 6.

A symbolic vote the Senate took on the measure last month passed by a 75-24 vote, including support from both Ohio senators.

"Anytime in today’s Senate that you can get people all across the political spectrum coming together and agreeing on something," especially by such clear margins, its chances are very good, said Betsy Laird, senior staff vice president of global public policy for ICSC.

"The timing is difficult to predict, but the outcome is fairly certain," she added.

If approved by the Senate, the measure would still need to be approved in the House before being sent to the White House, whose spokesmen have said President Barack Obama supports the bill.

Two Ohio Congressmen, David Joyce, Republican of Russell Township, and Steve Stivers, Republican of Columbus, are cosponsors of the House bill.

Proponents point out that states that don’t like the bill, perhaps because they don’t charge their residents state sales taxes, could opt out.

"The argument that we and all the other supporters of this legislation made is that this is really a state’s rights bill," Laird said. "It empowers the states to do what they want, and if the state doesn’t want to do this, it doesn’t have to."

Kercheval said that the fact that three-fourths of the Senate supports this helped give it greater credibility among members of the House.

"In talking to a number of congressman two weeks ago, many of them said to me . . . on both sides of the aisle, ‘If the Senate thought this was a good idea, we should take a look at this, too,’" he said. "The Congress members who’ve read this said, ‘This actually makes sense.’ "