SALT LAKE CITY — The pressure is increasing for lawmakers not to override Gov. Gary Herbert's veto of a bill earmarking 30 percent of future additional sales tax revenues for transportation.

Opponents of SB229, including the United Way and representatives of education and human services groups, are set to rally on the steps of the Utah Capitol on Wednesday.

"My sources have said the veto is not for sure," said Tom Love, United Way of Salt Lake board chairman and president of Love Communications. "We wouldn't be doing this if we felt we were wasting our time."

Love said the message of the rally is, "no more earmarks. Please do no not handcuff future legislative bodies." He said the amount the bill would set aside for roads is "a huge number" that puts other areas of the budget at risk.

That's the same argument the governor made in vetoing the bill last month. But the two thirds of the Legislature that asked their leadership to call the override session this Friday see the issue differently.

"I think there are many people who don't understand the bill," said Sen. Stuart Adams, R-Layton, the sponsor of SB229. "It's pretty complex."

Adams said the bill would increase the amount of sales tax revenues that go toward transportation projects about 10 percent, to slightly less than 25 percent. The 30 percent earmark of growth in sales tax revenues wouldn't begin until July 1, 2013, and is expected to last two to five years.

Even more confusing than the formula, Adams suggested, is the purpose of the set-aside.

"The first thing it does is fund transportation. If that were the only thing, that's probably not enough for the bill to have the type of support it has from my colleagues," Adams said.

He said the bill also is an alternative to an unpopular increase in the gas tax, which hasn't been raised since 1998. Adams said his own conservative constituents, for example, would balk at higher gas taxes.

"It's very, very difficult to get everyone on the same page on that issue," Adams said, noting the governor has at times backed a future gas tax increase but told reporters last week he would have also vetoed any such hike last session.

And the bill is being touted as something of a second Rainy Day fund, just as transportation funds already have been used in past sessions during the worst of the economic downturn, Adams said.

Sales tax collections, he said, are volatile, and there's too much temptation to spend the additional revenues on ongoing programs in good times, rather than on one-time expenses such as road construction.

"Even though we have a very conservative Legislature, that is, at times, impossible to say no to," Adams said. "Try telling someone you have the money and you're not going to give it to them."

But Adams said he disagrees the set-aside takes money that could go toward education and other state needs. "That is absolutely incorrect," he said, noting any increase in the sales tax also means more money for other programs, even with the earmark.

Love, though, said the lawmakers "who want this money taken off the table, don't want it to go to education and to other programs."

The United Way board chairman said the organization generally supports user fees as a source of funding, such as gas taxes, which are only paid by those who fill their tanks.

"Everyone is so afraid of raising taxes in a difficult economic environment and in a political year," Love said. "We will need to adjust the gas tax, at least to keep up with inflation. We shouldn't be afraid of that discussion."

Raising gas taxes to pay for transportation needs has long been the position of the Salt Lake Chamber.

"It's not an easy sell, but it's hard to argue against the fairness of it," chamber spokesman Marty Carpenter said.

Rather than holding a veto override session, Carpenter said the chamber is still hoping lawmakers will instead agree to study the transportation funding issue over the interim.

That would mean tackling the possibility of a gas tax increase during the 2012 Legislature, just months before most lawmakers and the governor are up for re-election.

"We are pro transportation. We understand the value of funding it," Carpenter said. "We have the luxury of thinking long term — we're not running for re-election."

University of Utah political science professor Matthew Burbank said he believes the public "would come down fairly heavily" in support of Herbert's veto because his concern for the impact on other state programs is easier to understand than the argument made for the override.

"I don't see why there's a real strong case to be made here and there's certainly the potential for problems," Burbank said. "While we know the Legislature loves roads and loves building them, they're also still politicians, and they know every time they come into session, there are competing priorities."

That could persuade some of the lawmakers needed to reach the two-thirds majority required for an override to rethink their vote, he said. "If they look at this bill all by itself, they might well be persuaded by the governor's point of view."