Capping a two-year effort by state legislators and six months of organizing, fundraising and grass-roots campaigning, Northern Virginia voters will go to the polls Tuesday to decide a critical issue for their region: whether to levy a regional tax for transportation.

The campaign to raise the sales tax by a half-cent has cost proponents millions of dollars and triggered a counteroffensive by anti-tax opponents that is every bit as passionate.

The biggest issue on the Northern Virginia ballot this Election Day does not involve a big-name politician but instead poses a key question to residents of the fast-growing Washington suburbs: Should they keep asking state government to meet their needs for new roads and public transit to ease traffic gridlock or steer a different course by taking charge themselves of solutions to problems of rapid growth?

The stakes are huge, for commuters, taxpayers, elected officials, businesses and economic interests such as the development community.

Voters in Fairfax, Prince William, Loudoun and Arlington counties, and the cities of Alexandria, Falls Church, Manassas, Manassas Park and Fairfax, will choose whether to boost the sales tax in the region to 5 percent from 4.5 percent, at a cost of about $94 annually for a typical household, according to state estimates.

Voters in the Hampton Roads area will vote on a similar measure that would raise the sales tax by a penny for transportation projects.

If approved, the higher sales tax would support about $5 billion in new road and transit projects in Northern Virginia over 20 years. Most of those projects would widen existing roads, build new ones or extend mass transit in the region's major corridors: Interstates 66, 95 and 395, the Capital Beltway, Route 1, Route 28, Route 7, the Dulles Toll Road and the Fairfax County Parkway.

For proponents, including Gov. Mark R. Warner (D), who has campaigned intensively for the measure's passage, the referendum represents a form of self-determination as state revenue plummets and state lawmakers prepare to cut hundreds of millions of dollars in spending. A tax increase, they say, is the Washington suburbs' only chance to guarantee a steady stream of road-building money that would stay in Northern Virginia, rather than be diverted to the state treasury for other needs.

"We all sit in traffic," said John G. Milliken, a Warner ally and the chairman of the pro-sales tax group Citizens for Better Transportation. "We know we have a problem, and this is the best way to address it."

Milliken, a former state transportation secretary, appealed to the frustration felt by many in Northern Virginia that the region is sending so much money to Richmond to support other parts of the state that it cannot meet its own needs.

"Finally, we can take this matter into our own hands," Milliken said. "All the money that's raised up here will be kept up here, and the decisions will be made by people up here."

Opponents describe the referendum as a turning point for a region that needs to make hard choices about how public dollars are allocated. Homeowners already angry over rising property tax assessments have revved up a tax revolt that has drawn support from local Republican organizations from Arlington to Prince William County.

Joining the anti-tax forces are "smart-growth" advocates, who say that adding lanes to the region's roads would increase suburban sprawl by encouraging more development around those roads. Then, instead of reducing congestion, the so-called improvements funded by the sales tax increase would actually make traffic only worse, they argue.

Opponents also argue that by taxing themselves, Northern Virginians would let the state's elected leaders off the hook. Instead of raising the sales tax, local politicians should work harder to receive a larger share of the state tax money the region sends to Richmond, they say. Otherwise, state planners will argue that they do not need to spend money on the Washington suburbs if Northern Virginia has its own stream of revenue.

"Our number one case against this is, it is entirely possible that if we pass a tax hike, in the end we will have fewer transportation dollars in the long haul," said James T. Parmelee, a spokesman for the Northern Virginia Coalition to Stop the Sales Tax. "The pressure will be taken off Richmond to return more money from rural to suburban areas."

Some projects that would be built with proceeds from the tax increase are still in the planning stages, and many depend on winning federal money, leading opponents to criticize the plans as merely a wish list with no guarantees.

Opponents and advocates for the tax have debated the issue since Labor Day at school forums and before civic associations and business groups. A majority of Northern Virginia's delegation to the General Assembly supports the tax increase, although opponents won a legislative victory in August when Ken Cuccinelli (R), a Centreville lawyer who made his opposition to the tax the centerpiece of a special election campaign, won a seat in the Virginia Senate.

The new sales tax money would be spent by the Northern Virginia Transportation Authority, a newly created board of local government appointees. Advocates say this strengthens their case that the region's transportation needs would be addressed by local decision-makers, rather than by planners in Richmond. Opponents have expressed concern that smaller jurisdictions such as Arlington would be shortchanged by the needs of larger ones such as Fairfax.

The pro-tax campaign enjoys a big financial advantage over that of opponents. Citizens for Better Transportation had raised about $1.5 million by the end of September and reported having about $722,000 in the bank. Opponents reported no fundraising, but they are supported by well-established anti-tax and slow-growth groups that have refused to report their fundraising for the tax campaign, citing the complexity of separating out money they raise for other causes.

A debate on the proposed half-cent tax increase packed this hotel ballroom in September.