Marshall Coleman just can't keep a secret.

This week, the GOP gubernatorial candidate sent thousands of Virginia Republicans a "confidential" campaign memorandum that he said in an accompanying letter had been "prepared just a few days ago."

In fact, the "leak" was one of the latest tactics Republicans have used in a costly postal war with Democrat Charles S. Robb, which campaign strategists say could decide who is Virginia's next governor.

Stamped "Confidential" in the upper right-hand corner and "Received Sept. 22" in the left, the purported inside look at Coleman's campaign concluded, not surprisingly, that things couldn't be better.

The first hint that the report was meant to supply other than privileged information came in the sixth paragraph of the Coleman letter, when he said the "good news" of the report was "tempered by our very legitimate concern about our need to raise the funds" to continue the campaign.

Direct-mail experts call this technique "involvement," or luring people into a belief they are sharing a secret.

Four years ago, Democrat Henry E. Howell complained that he lost his bid for governor because of John N. Dalton's superiority in winning voters through the mails. But for the first time in a statewide race Democrats also are waging a massive mail campaign to woo Virginia voters.

Coleman already has spent an estimated $300,000 flexing his computer muscles in direct-mail appeals. Before the race is over, the two campaigns will have spent close to $1 million -- apart from their multimillion-dollar media efforts -- combining hundreds of thousands of telephone calls, computerized voter profiles and crafty letter-writing in efforts to stir Virginians through their mail boxes.

Still, with more money and more experience, the Republicans are expected to win this battle. Despite the Democratic efforts, the differences in direct-mail abilities between the two parties are "dramatic," says University of Virginia political scientist Larry Sabato.

"Democrats for a long time have been fat and happy with the old ways," Sabato says. "They just didn't spend the time or money developing it."

Republicans have developed a bank of computer tapes that tell them how Virginians have voted in recent years. Democrats, who have never had the money to build voter profiles, are just getting into the game.

"We know what's available and we know what works," says Russell Rosen, newly installed executive director of the state Democratic Party. "If no one has really laid out for you the advantages and the disadvantages, then obviously you're not going to use it or develop it as part of your strategy. And that has been the case in the past."

The name of the game in direct mail is knowing what is on voters' minds, and giving them a precise message. In the last governor's campaign, for instance, Dalton sent letters emphasizing local issues to those living in different parts of the state.

In an upcoming mailing of 200,000 letters for Robb, campaign strategists plan to send three separate messages: One to voters with strong Democratic preferences who usually vote, one for those who vote less often, and one for "swing" voters who will vote for either party. These "swayable" voters, both sides concede, will be crucial in the Nov. 3 election.

The possibilities for sending various messages to people are practically endless, says Glen Cowan, vice president of National Voter Contact, the Washington-based political marketing firm producing Robb's direct-mail effort.

The reason is the computer. As the election draws near, Democrats and Republicans will make hundreds of thousands of telephone calls -- 550,000 is the Coleman campaign goal alone -- to identify voter preferences. The information is forwarded to computer programmers who classify voters according to likes, dislikes, incomes and geographic location and match them with blinding computer speed to letters custom-tailored to inspire support.

Once that information is programmed onto tape, laser printers turn out personalized letters at the rate of more than 7,000 an hour.

"We know generally where they voters are so that we can be 85 percent accurate in getting the kind of profile we're looking for," says Republican direct-mail expert Ed DeBolt of Arlington. "We can mail to that person with quite a bit of assurance that he's going to fit the profile and be receptive to that kind of message."

Last week, the computerized printer on the fifth floor of a Richmond marketing firm was chattering away day and night, spitting out a letter from Ronald Reagan that was designed to inspire a large Republican turnout and capture some of the "swing" voters Coleman needs to win.

Though it was written by a machine more than 100 miles from Washington, it was imprinted mechanically with Reagan's signature and urged voters to elect Coleman as a measure of support for the president's programs. The computer addressed each letter, giving the impression that the president had personally addressed them from the Rose Garden.

"If they voters are not fooled," says Sabato, "they're at least entranced. People are television educated, but they're not yet direct-mail educated."

The reason direct-mail is effective, experts say, is because people believe what they read even more than what they see on television or hear over the radio. "It's just like people believing the newspaper," says DeBolt.

To get people to open the letter and read it, direct-mail writers look for a "personal" effect, "as if the guy were writing correspondence to his family or his best friends," says Phil Smith, former director of mail advertising for the Republican National Committee.

Stationery is simple rather than artistic, as if it came from the candidate's desk at home. Signatures are reproduced in colored ink. Simply placing a stamp on the envelope, rather than a metered franking, can mean a 5 percent better chance the letter will produce results.

Often letters end with a handwritten postscript that looks as if the candidate wrote it -- and initialed it -- himself. The theory is that people first read the beginning and end of the letter, and want a capsule summary of what it says.

"Please let me know right away," writes Robb at the end of a recent fund-raising appeal, "if I can count on you -- and thank you for caring."

Candidates also use political mailings to hit hard at each other, because the letters attract relatively limited media attention. The Reagan letter takes a swipe at the Great Society, whose liberal policies Coleman has been trying to pin on Robb. Previous letters from the Coleman campaign appealed for help to counter money from Robb's "Texas friends." The computer cranked out two letters on the subject identical in every respect except one: Coleman signed one letter, Dalton signed the other