For the second year in a row, terror took the politics right out of Northern Virginia's political season.

Like the Sept. 11, 2001, attacks on the Pentagon and the World Trade Center, the Washington area sniper killings this month pushed virtually all electioneering off center stage. With daily routines turned upside down, the last thing on most voters' minds was the smattering of local and statewide races -- or even the high-stakes regional campaign over raising the sales tax for transportation.

"All of a sudden, voting went way down on the priority list," said Toni-Michelle Travis, a Virginia politics expert at George Mason University in Fairfax County. "It's not on the radar. It's been lost in people worrying about the sniper night and day."

Even as the region breathed a collective sigh of relief after the arrest of two suspects late last week, Travis and other experts said they fear that residual jitters may depress voter turnout in a year when participation was expected to be light anyway. Polls are open Tuesday from 6 a.m. to 7 p.m.

"It's going to take a while for us to come back to a sense of normal," said state Del. John A. "Jack" Rollison III (R-Prince William), a leader of the bipartisan coalition pushing the sales tax increase. "I'm confident our region will regain our sense of balance -- and that includes voting -- but I don't think we'll ever be 100 percent."

The off-year election cycle seemed to be favoring incumbents, notably U.S. Sen. John W. Warner (R), who is campaigning for a fifth six-year term against two independent candidates; state Democrats declined to nominate anyone in the race.

Similarly, the three members of Congress who represent most Northern Virginians -- Democrat James P. Moran of the 8th District and Republicans Frank R. Wolf and Thomas M. Davis III of the 10th and 11th Districts, respectively -- face little-known opponents, though Moran's recent ethics controversies created an issue for his GOP challenger, Scott C. Tate.

Turnout has been the all-consuming objective in the campaigns on the proposed transportation sales tax. Both sides were using direct mail, telephone banks and door-knocking by volunteers to identify and mobilize their respective bases of support across a region as politically diverse as any in the Old Dominion.

Gov. Mark R. Warner (D), a Northern Virginian who endorsed holding referendums in his gubernatorial campaign last year, took a lead role this fall in advocating the higher sales tax. The Democrat has staked the prestige of his office and a sizable share of his executive clout on winning passage of the tax increase on his home turf and on an even more ambitious measure in the sprawling port region of Hampton Roads.

To pass in Northern Virginia, the tax increase proposal must secure a regionwide majority in the counties of Arlington, Fairfax, Loudoun and Prince William and the cities of Alexandria, Fairfax, Falls Church, Manassas and Manassas Park. The Washington area proposal would raise the sales tax from 4.5 percent to 5 percent, which would generate an estimated $5 billion over 20 years for mass transit and highway improvements. Hampton Roads voters are deciding on a proposed 1 percent increase in the sales tax rate.

As in Tidewater, the political struggle in the Washington suburbs has been cast as a kind of David vs. Goliath struggle between a loosely organized and poorly financed band of opponents -- "peasants with pitchforks," in the words of one of them -- and the titans of commerce and real estate development who are largely underwriting the $2 million cost of the proponents' campaign.

The biblical analogy isn't entirely apt. The opponents include some of the best strategists of the conservative Republican revolution that has quietly transformed the region's politics in recent decades. The GOP has won a string of elections with attractive candidates and their networks of highly disciplined volunteers, many of them Christian activists who strongly oppose abortion rights.

In August, opponents of the transportation tax claimed an important victory when lawyer Ken Cuccinelli (R) won an open seat in the state Senate in western Fairfax County. Many Cuccinelli supporters are now working for the state Senate campaign of Del. J.K. O'Brien Jr. (R-Fairfax), whose race against Democrat Rosemary M. Lynch could produce an important voter surge in their Fairfax-Prince William district at the center of the referendum region.

The special election between Lynch and O'Brien is the region's only state legislative contest this fall. Regular General Assembly elections are next year.

Proponents of the sales tax increase enjoy an overwhelming advantage in money, which could be crucial in mobilizing voters -- and running television commercials -- in the campaign's final days. Advocates also have the endorsement of such key groups as labor unions and PTAs that are encouraging their members to support the tax proposal.

Voter surveys by the proponents and independent pollsters have consistently shown majority support for the sales tax proposal, though the margin seems to have slipped somewhat in recent weeks. Both sides agree that although the surveys reflect the region's deep frustration with long commutes, the findings may be all but meaningless depending on who actually goes to the polls.

There are just under 1 million registered voters in the region, and experts on both sides are predicting turnout of less than 50 percent, about typical for a year without a presidential election. At this point, both sides are feverishly courting a core group of undecided voters, who might end up providing as much as 20 percent of the vote.

The undecideds have left both sides of the referendum proposal guessing about ultimate turnout, which the two camps agree could be affected by the three weeks of sniper slayings.

"There was a lot more focus on the sales tax before the sniper. Voters have been literally very distracted," said Stewart Schwartz, executive director of the Coalition for Smarter Growth, which opposes the transportation tax on environmental grounds. "Even if it's a depressed turnout, it's hard to predict."