JEFFERSON CITY — Opponents launched a campaign Tuesday against Missouri's proposed transportation sales tax by raising concerns about the way the August ballot measure would raise and spend hundreds of millions of dollars annually.

The new opposition group, Missourians for Better Transportation Solutions, said the proposed three-quarters cent sales tax would mark a "radical departure from the way roads have been traditionally financed" and could affect lower-income residents more than some of the heaviest users of state highways.

"Using the sales tax to fund roads is really a bad idea," said Thomas R. Shrout Jr., a St. Louis consultant who is treasurer of the opposition group.

Missouri roads historically have been funded by user fees, such as state and federal motor fuel taxes and state sales taxes charged on vehicle purchases. If approved by voters, proposed Constitutional Amendment 7 would mark the first time that general sales tax revenues have been earmarked for highways.

The proposal is estimated by lawmakers, who referred it to the ballot, to raise $534 million annually for the next 10 years. It's projected by Gov. Jay Nixon, who announced his opposition to the measure last week, to raise about $575 million in its first full year and $6.1 billion over the next decade. Either way, the tax increase would be the largest in state history.

Shrout said the sales tax would impose a burden on people living on fixed incomes, such as retirees, while requiring virtually nothing from truck operators who carry their cargo through Missouri. He said either a fuel tax increase or toll roads would be fairer.

Supporters have said that a sales tax could generate more money than a fuel tax and has proved more popular in public opinion surveys.

"Amendment 7 is the only sustainable solution to fix our roads and bridges and create jobs," said Bill McKenna, a former state transportation commissioner who is treasurer of the supporting group, Missourians for Safe Transportation and New Jobs.

Supporters had $343,000 in their campaign account as of the end of March and have continued to raise money. On Tuesday alone, the campaign committee reported receiving more than $120,000 of contributions.

McKenna said voters "can expect a full-fledged public education effort" from the coalition supporting the tax proposal.

Shrout said he doesn't expect opponents to raise enough money to broadcast advertisements. He said they likely will rely on social media and public events.