Democrats are watching the roiling transportation debate and thinking: The bill for Republican Gov. James S. Gilmore III's car-tax rebate is finally coming due.

But it remains largely an unspoken thought. Gilmore's car-tax plan will drain billions of dollars out of the state budget over the next few years at a time when heavy traffic in Northern Virginia and some other parts of the state is being declared a crisis.

Yet as eagerly as Democrats have sought to claim the transportation issue, they are avoiding talk of taxes like a burned child avoids a stove. The thought of running a campaign on that issue--and risking a replay of 1997's thrashing at the hands of Gilmore and his "No Car Tax" slogan--is too painful for many Democrats to contemplate.

The cost of the car-tax cut this year is $300 million. The annual cost will reach $1.1 billion by 2002, when the plan is fully phased in, and it will keep growing. That's more than double the annual dollars proposed for even the most ambitious plans to ease traffic.

"Maybe [three years] from now, people will say, 'This is just the dumbest thing we've ever done,' " said Del. Barnie K. Day, a Democrat from rural Patrick County near the North Carolina border, who wonders whether people stuck in traffic jams would give up the tax savings for a quicker commute.

Day is one of the few speaking out on the issue, though both Democrats and Republicans agree on its essential logic: Tax cuts are trade-offs between dollars in your pocket and dollars for government services. The bigger the tax cut, the bigger the trade-off.

The car-tax cut, which Gilmore bills as the biggest tax break in the state's history, is causing some like-sized problems for the search for new transportation money, lawmakers say.

"Oh yeah, no question about that," said Del. Vincent F. Callahan Jr. (R-Fairfax).

Where Democrats and Republicans split is on the question of whether the car-tax cut is worth the headaches. Republicans like Callahan say they have no regrets in supporting it. "It was a horrendous, heinous tax that people hated," he says.

A typical Democratic reaction comes from state Sen. Richard L. Saslaw (D-Fairfax), who said: "The equivalent of the car-tax money would allow us to fund all these projects around the state."

"But the voters made a choice," Saslaw added. "You can't do both. . . . It's already over. It's in concrete."

Voters weren't the only ones who made a choice. Saslaw and other Democrats voted reluctantly but unanimously for the car-tax cut as part of a deal to get state funding for school construction. That now makes it harder to attack the tax cut, Democrats say.

The issue of taxes has long lurked just below the surface of the transportation debate.

Leaders of both parties have crafted their proposals so that new taxes aren't needed, but Gilmore has been eager to portray those calling for huge new spending on transportation as closet taxers.

That has allowed him to portray himself again as the tax cutter, a role that propelled him into office and helped Republicans sweep the top state offices and win the Senate in 1997. That victory echoed in car-tax bills received in Northern Virginia households this month that detail the amount of tax savings this year and direct questions to "Governor Gilmore's No Car Tax Hotline at 1-877-CARTAX1."

Northern Virginians benefit disproportionately from the car-tax cut because residents are more likely to drive expensive cars. Last year, 38 percent of the car-tax refund dollars went to Northern Virginia. The average driver in Fairfax got back $42.71. Eliminating the tax altogether, which is scheduled to happen in 2002, will save many in the Washington suburbs hundreds of dollars a year.

In a speech to legislators Monday, Gilmore blasted several Democratic ideas for easing traffic problems. But he spent nearly as much time touting the car-tax cut and 15 other forms of relief that will eventually send $1.5 billion a year back to taxpayers.

And on his monthly radio show Thursday, Gilmore attacked Northern Virginia business leaders for proposing raising the sales tax in the region by 1 percentage point, calling it a "dumb, stultifying, old-time approach to things."

Gilmore plans to announce a transportation package on Tuesday that would bring about $2 billion in new spending, with about half that going to Northern Virginia. But administration officials say there will be no talk of new taxes.

Gilmore spokesman Mark Miner said that Gilmore, blessed with a roaring economy, has shown he can cut taxes while still boosting spending on mental health, higher education and public schools. The governor will do the same on transportation, Miner said.

"The governor believes that the taxpayers need to be a priority when governing," he said.

With November elections putting every legislative seat and control of the General Assembly at stake, lawmakers are avoiding tax proposals as well.

But some suggest Northern Virginia's transportation problems show the limits of tax-cutting: Voters love it, until they want new spending.

University of Virginia economist John L. Knapp said that Virginia was a low-tax state--48th out of 50 by one measure--in 1996, even before Gilmore's car-tax cut plan took effect. Cutting that burden even deeper, he said, means sacrificing spending on services.

"There's no controversy about that," Knapp said. "If you cut taxes, then you're going to have less money in the future."

The leading Democratic proposal on transportation would bring $2 billion over four years by using anticipated state surplus and borrowing against both the state's share of the national tobacco settlement and the state's real estate transaction tax.

Northern Virginia Republicans, led by Del. John H. "Jack" Rust Jr. (R-Fairfax), are pushing for $3.5 billion over seven years by borrowing against the state's general fund.

But these proposals would just scratch the surface of Northern Virginia's traffic problems, experts say. Rust's plan and the Democratic one would add $500 million a year statewide.

That wouldn't even be enough to help Northern Virginia, which alone needs $600 million a year in new spending, according to the Greater Washington Board of Trade. The Northern Virginia Transportation Coordinating Council says it will take $11 billion to just keep traffic from getting worse by 2020.

By that time, the car-tax cut will have saved Virginians more than $20 billion.

"Folks have to think if the pain of the traffic congestion . . . is worse than the pain of not getting the money back in their pocket," said Robert T. Grow, transportation director for the Greater Washington Board of Trade.

CAPTION: Gov. James S. Gilmore III (R), left, shown with Attorney General Mark L. Earley (R), plans to announce a new transportation package next week.