By Matt Murphy

State House News Service

BOSTON -- Attorney General Martha Coakley's botched handling of a question about the state's gas tax over the weekend elicited stinging criticism Monday from Republicans as the front-running Democrat for governor characterized the gaffe as a simple mistake.

Coakley, appearing on WCVB's Sunday-morning news program "On the Record," incorrectly guessed that the tax charged by the state on every gallon of gas sold in Massachusetts was 10 cents.

The state actually taxes each gallon of gas 24 cents, and the gas tax was the subject of considerable public debate last year when the Legislature voted to increase that tax by 3 cents per gallon and tie future increases to inflation as part of a plan to finance transportation-infrastructure improvement.

The gas tax was also at the center of debate in 2009 when Gov. Deval Patrick unsuccessfully tried to raise it by 19 cents a gallon.

"Very few people get 100 percent of the pop-quiz questions on OTR correct -- and Martha made a mistake on this question. Martha knows that the gas tax is a critical funding source to make the transportation-infrastructure investments that are necessary to move Massachusetts forward," Coakley spokeswoman Bonnie McGilpin said in a statement to the News Service.

Opponents, however, were quick to use the flub to portray Coakley as out of touch with voters. It's the type of characterization Coakley's campaign has been trying to avoid as it works to rebuild her image after the attorney general's failed bid for U.S. Senate in 2010 when she faced similar criticism.

"If you want to be governor, and you support an automatic increase in the gas tax, you should know what the current tax rate is," Republican gubernatorial candidate Charlie Baker said Monday morning in a statement. "That the attorney general, who had to certify a ballot question on this topic, would think the state gas tax is 10 cents is a little scary."

Later in the afternoon, Baker's campaign sent out an emailed fundraising appeal titled "You Won't Believe This" with a link to the video clip of Coakley's television appearance.

Coakley's office vetted and certified a potential ballot question for November that would repeal the new law indexing the gas tax to inflation. Coakley, one of five Democrats running for governor, does not support repeal.

Baker said the mistake showed Coakley to be "out of touch" and "uninformed," while Rep. Geoff Diehl, a Whitman Republican and organizer of the gas-tax ballot question campaign, said the gas tax hasn't been 10 cents per gallon since before he was born.

"How does she not know what drivers are paying? Talk about being out of touch," Diehl said. "Of all people, she should know that the state gas tax is 24 cents per gallon, with an additional 2.5 cents for underground storage, with another 18.4 cents per for federal taxes. We pay over $7 in taxes for a 15-gallon fill-up."

A spokeswoman for Treasurer Steven Grossman, one of Coakley's leading rivals for the Democratic Party nomination for governor, also took a jab at the attorney general.

"Martha Coakley is clearly out of touch with our economy and our state's finances. If you want to be governor, you should know what the people of Massachusetts are dealing with every day," Chandra Allard said.