For the first time in nearly two decades, Virginians will be presented with the following choice when they walk into their polling places Tuesday: Do they want a Democratic or a Republican ballot?

The last time the two major parties held a statewide primary on the same day was in 1988. In Virginia, voters do not register as Republicans or Democrats and may choose to vote in either party's primary -- but not both.

The answers voters give when asked which ballot they want will dictate who they can vote for and might shape the results of close elections where middle-of-the-road candidates are hoping to draw support from independents and supporters of the other party.

Larry J. Sabato, director of the Center for Politics at the University of Virginia, said centrist Republican lawmakers battling conservative challengers may get fewer Democrats voting for them because of the Democratic primary for lieutenant governor.

"They would have gone in for [Del. Gary A.] Reese and [Del. Joe T.] May and the other ones," Sabato said of Democratic voters. "But probably they will vote for lieutenant governor in the Democratic primary. Boom. They lose their opportunity to vote for the moderate Republicans."

In 1996, U.S. Sen. John W. Warner (R-Va.) won the support of many Democrats in a primary against James C. Miller III. But Sabato noted that there was no Democratic primary that year to compete for the attention of voters.

"Warner had loads of Democrats," Sabato recalled.

Election officials say they expect light turnout for Tuesday's primaries. Jean R. Jensen, secretary of the State Board of Elections, said she predicts that about 10 percent of the state's eligible voters -- 400,000 people -- will cast ballots.

But Jensen said there might be more confusion than usual, and a bit of frustration, at the polls because of the dual primaries.

When voters walk into a polling place, she said, poll workers will provide sample ballots for both party primaries. They will then ask voters which party primary they want to vote in. If voters get to the ballot box and realize they want to vote in the other primary, Jensen said, they will be allowed to switch if they have not yet marked the ballot.

"They can go back, get in line. We'll have to correct the situation in the poll book," she said. "We're hoping that we're going to do enough educating and hand-holding that we can prevent that."

Jensen said she expects some frustration among people who want to vote for candidates in both parties, but she said state law does not allow poll workers to make exceptions.

"They might want to vote for somebody at the state level who's of one party and somebody at the local level that is of the other party," she said. "There's no splitting. You must choose one or the other."

Candidates from both parties said their volunteers will spread out among many of the state's 2,200 precincts, handing out literature and trying to keep track of the voters who show up.

Among the most sophisticated operations will be the one mounted by former attorney general Jerry W. Kilgore, who is running for the GOP gubernatorial nomination against Warrenton Mayor George B. Fitch.

Kilgore is running what his campaign calls "Operation Dry Run," which targets 100 precincts for intensive get-out-the-vote efforts, according to campaign manager Ken Hutcheson.

Hutcheson said the campaign has been trying for three weeks to drive up the filing of absentee ballots in those precincts and will try to boost turnout Tuesday. Later, the campaign will compare the turnout in those precincts with the results in precincts that were not targeted.

That will allow the Kilgore campaign -- if it wins Tuesday -- to fine-tune turnout plans for the general election Nov. 8 against Lt. Gov. Timothy M. Kaine (D), Hutcheson said. Kaine is unopposed for the Democratic nomination.