Traffic on the 495 Beltway is backed up from Bethesda to College Park due to emergency pothole repairsin 2010. (Photo by Linda Davidson/The Washington Post).

Maryland Gov. Martin O’Malley’s transportation funding plan is scheduled to get its first hearing Friday afternoon, but advocates on both sides weren’t waiting for testimony to begin voicing their opinions.

In a surprise move, AAA Mid-Atlantic, a motorists’ association that claims nearly 1 million members in Maryland, announced it would oppose the plan as currently crafted at the hearing by the House Ways and Means Committee.

The proposal — expected to yield $3.4 billion over the next five years — relies in large part on a new tax on gasoline to shore up a state transportation fund projected to run out of money for road and rail projects. And it has the backing of the two Democratic leaders of the General Assembly.

But while acknowledging that the new money is “desperately” needed, AAA said the plan has “major flaws,” including heavy use of tax revenue from drivers to pay for mass transit projects.

“Requiring motorists to pay virtually all of the proposed new revenue is not something we as a motorists’ organization think is fair and can support,” said Mahlon G. “Lon” Anderson, an AAA spokesman.

More predictably, the Maryland Republican Party also announced its opposition, questioning the need for another tax increase from Annapolis at a time of high gasoline costs. “When will the insanity stop?” Diana Waterman, the party chairwoman, said in a news release.

The proposal got a far more favorable reception at a morning news conference, where business and labor leaders from both the Washington and Baltimore regions offered their strong support for raising new revenue.

“Nothing is more important as it relates to keeping business going,” said Jim Dinegar, president of the Greater Washington Board of Trade.

An environmental group, 1000 Friends of Maryland, also weighed in on behalf of the governor’s initiative in a statement released Friday morning.

“The Maryland Transportation and Infrastructure Act will expand much-needed public transit for our citizens even as we protect existing investments in roads, bridges and highways,” said Dru Schmidt-Perkins, the group’s executive director.

O’Malley’s plan would trim 5 cents off the state’s existing gas tax of 23.5 cents a gallon. The lost revenue would be more than offset, however, by a new 4 percent sales tax on the wholesale level, phased in over two years. The flat gas tax also would rise in future years to reflect inflation.

Motorists could expect to pay about 2 cents more per gallon of gas starting in July and an additional 7 cents more a year after that, O’Malley administration officials have said.

Maryland has no money budgeted for new highway construction after 2017. And no funding has been identified to pay the state’s share of long-planned rail projects.

Without new funding this legislative session, the state Department of Transportation plans to halt design work on the Purple Line, the 16-mile light rail line that would connect Bethesda and New Carrollton, as well as on a dedicated express bus route in the Interstate 270 corridor.