Critics of a law that allows for automatic increases in the state's gasoline tax have pounced on the issue with the hope of pushing it to the forefront of the election year debate.

Republicans, currently shut out of statewide offices and heavily outnumbered in the Legislature, have embraced the fight against gas tax indexing and believe their cause could resonate strongly with mainstream voters, particularly independents and moderate Democrats, whom the party desperately needs for success in November.

The Democrat-controlled Legislature last year raised the gasoline tax by 3 cents to 24 cents a gallon -- the first increase in the tax since 1991 -- as part of a broader transportation finance bill. Lawmakers also indexed the tax to inflation, meaning it will automatically rise by the same annual percentage as the U.S. Consumer Price Index.

Charlie Baker, the GOP's endorsed candidate for governor, said the automatic hikes absolve lawmakers of responsibility for voting on future tax increases and amount to "taxation without representation." Baker and other Republicans support a statewide ballot question that would repeal the indexing provision.

Peter Ubertaccio, a political science professor at Stonehill College, said it was a potentially thorny issue for Democrats.

"Whenever you have the words `automatic' and `tax increase' in the same sentence, you're already on the defensive," he said.

Supporters of indexing say it's necessary to ensure the state has a reliable stream of revenue to pay for critical road and bridge repairs and to finance key transportation projects such as SouthCoast rail and the extension of the MBTA's Green Line.

"It's irresponsible for Charlie Baker to oppose a critical source of transportation funding without offering any plan to replace it," said state Treasurer Steven Grossman, one of five Democrats running for governor.

The gas tax differs fundamentally from the state sales tax because it is a fixed amount rather than a fixed percentage of the purchase price -- meaning revenue from the tax doesn't automatically go up as gas prices rise, proponents note.

With the CPI averaging about 1 or 2 percent in recent years, the tax would likely rise half a penny or less a year.

"It's a tiny increase," said Michael Widmer, president of the nonpartisan Massachusetts Taxpayers Foundation. Yet it would generate significant revenue to help the state keep up with road and bridge maintenance, he said.

Widmer said it was politically unrealistic to expect the Legislature to revisit the gas tax on its own anytime soon, given that it hadn't touched the tax in more than two decades.

Stop Automatic Tax Hikes, a group that petitioned for the ballot question, warns that if gas tax indexing stands, lawmakers would be emboldened to do the same with other taxes to avoid difficult votes.

Joseph Avellone is the only Democratic gubernatorial candidate who opposes indexing.

"As governor, I would be transparent and upfront with the public when advocating additional taxes on the middle class and small business," said Avellone, a business executive who has positioned himself as the most moderate of the Democrats in the race.

Attorney General Martha Coakley drew sharp criticism from Baker and others after incorrectly guessing during a recent TV interview that the current gas tax was 10 cents. A campaign spokeswoman later said Coakley fully understands the importance of the tax in funding the state's transportation infrastructure.

Democratic hopefuls Don Berwick and Juliette Kayyem also support the current law.

Democratic Gov. Deval Patrick, who is not seeking re-election, noted that the state's income tax is automatically reduced by a small amount when revenues exceed certain benchmarks.

"We index the income tax. It's gone down on account of that indexing two times since I've been in office," Patrick said. "None of the Republicans have complained about that."