The days had dwindled down to a precious few, and Dan Crane was growing increasingly nervous as he looked ahead to Nov. 7. It was time, he decided, to play the ace.

On paper, Crane should have been brimming with confidence about his campaign for the open House seat here in Illinois' 22nd Congressional District. The intense but friendly Republican had amassed a huge campaign treasury ($299,000), and his polls gave him a thin lead over his Democratic opponent, Terry Bruce. Most important, Crane had effectively spread the word among the 22nd's conservative electorate that he was the more conservative of the two candidates.

In the past two weeks, however, the Crane campaign had endured enough reverses to make Pollyanna take up nailbiting.

Crane had been criticized sharply for backing out of a televised debate with his opponent. He had been stung by Bruce's hard-hitting radio ads, which quoted an offhand comment Crane had made months ago suggesting termination of the Social Security Administration. And he had watched unhappily as the 22nd's newspapers, in editorials endorsing Terry Bruce, had described Crane's right-wing philosophy as "doctrinaire" and "radical."

Clearly, the situation called for heavy artillery. So Crane called Richard Viguerie, the direct-mail guru in Falls Church, Va., and told him to launch The Wife Letter.

The Wife Letter, a "personal" message from a candidate's wife to a selected group of voters, is the piece-de-resistance of the elablorate system of post office politics that Viguerie has designed for conservative candidates around the country.

"The Wife Letter," explains William Radigan, a top executive in the Viguerie organization, "is the most potent single piece of mail in American political history. I mean, that thing works like gangbusters. I don't know who thought it up - probably Viguerie, he's the genius - but every place we use it, that wife mailing is the most effective thing that anybody's ever seen."

Although Viguerie is best known for the success of his direct mail fund-raising drives, his computerized letter-writing operation also grinds out missives for other purposes, such as convincing public officials to adopt certain positions, or convincing voters to elect certain candidates. The Wife Letter falls in the last category.

The genius of sending voters a chatty, amiable letter from a candidate's wife (Viguerie has not yet tried a letter from a female candidate's husband) lies in the recognition that family values generally count more with the voters than political stands on particular issues.

That was reflected in a poll of the 22nd District commissioned last winter by Gene Stunkel, who ran unsuccessfully against Dan Crane in the GOP congressional primary. The survey showed that 58 per cent of the people here prefer candidates who are "conservative"; 63 percent favor those who have experience in public office; 72 percent like candidates with a background in business. But 95 percent of the voters said they would like their congressman to be a "family man."

Another explanation for The Wife Letter's success is that it is carefully camouflaged to remove all the telltale signs of mass mailing.

Viguerie requires that The Wife Letter be written out in longhand by the wife herself. The manuscript is then shipped to Viguerie's Falls Church headquarters where it is reprinted, still in longhand, on ladylike stationery, and stuffed in a personalized envelope, preferably of some pastel hue. The envelopes are stamped with real postage stamps - postage meters are too businesslike for this purpose - and shipped back to the candidate's district to be mailed from the post office nearest his home.

All of this makes The Wife Letter something of a Rolls-Royce among political mailings. With bulk rate postage (8.1 cents) and computerized printing, a normal mass mailing costs about 15 cents per letter. The Wife Letter, with its 15-cent stamp and its complicated photo-offset printing, is more than twice as expensive.

But the return seems worth the extra investment.

"The wife mailing really works because it is personal," Radigan explained recently. "More personal than any radio or TV ad. It's a personal, handwritten letter from the wife."

To preserve the personal aura of the correspondence, Viguerie dictates that The Wife Letter should be heavy on family chit-chat and relatively light on issues. Judy Crane followed those rules carefully last winter, during the 22nd's primary, when she went a personal letter, via Viguerie, to about 50,000 of the district's Republicans.

"Dear Friend," Judy's letter said, ". . . with the family and all (we have four small children and are expecting our fifth in July) . . . I haven't had too much time to myself. But I made up my mind today to sit down and write you . . ."

For four pages the letter went on, explaining Dan's concern, as a family man, about inflation, taxes, energy, and farm problems.

After the discussion of issues, however, Judy got back to family matters. "The baby's crying so I must close for now," she wrote at letter's end. "P.S. If you would like to chat with me about Dan's campaign, please feel free to call me at home at 217-443-0885."

The letter, and the picture of Dan's family that was enclosed with it, were a big hit among primary voters. Stumpling farm towns on the Saturday before the primary, Dan was approached time and again by people who said "I got a real nice letter from your wife," or "You have a real nice family - thanks for the picture."

The effet was incredibly long-lived. When Dan was campaigning in Pana, near the 22nd's southwestern corner, on Labor Day weekend - six months after the letter was mailed - voters were still telling him how much they enjoyed hearing from Judy.

The letter was so helpful in the primary that Dan knew all along he would want to use it in the general election as well. The only question was timing. There was some feeling within the Crane camp that it should be saved for the campaign's last weekend.Eventually, though, it was decided that the letter would go, to 100,000 selected voters (representing one of every two households in the district) this weekend.

The Wife Letter would reach the voters Monday or Tuesday, giving them a week to talk about it before voting day. And that might help deal with a serious problem facing both Dan Crane and Terry Bruce: getting people interested in the election.