As the mercury climbed toward 100 last Friday, the Coffey family of Alexandria zeroed in on a neighborhood in Kettering, Md., knocking on door after door.

By day's end, they returned to their Mercedes-Benz triumphantly. It had been an exceptional day: only 18 of the 20 households they had contacted turned them away in mid-sentence.

Jim and Naomi Coffey spend many of their weekends this way -- like 10,000 other people in the Washington metropolitan area, they are Jehovah's Witnesses.

The Witnesses were out canvassing neighborhoods last Friday as part of their annual district convention at the Capital Centre. They'll be back out again today, and by the end of their two-part, four-day convention, they expect to have contacted more than 50,000 area households.

The increasing sophistication of the Witnesses' well-traveled door-to-door campaigns -- or "field service," as they call it -- has helped their worldwide membership nearly double over the past decade to 2.2 million persons. Similarly, in the United States, their membership has grown from 343,000 to 570,000 over the same period.

The majority are adults who converted to the fundamentalist religion after their homes were canvassed.

Jehovah's Witnesses take their name from the Old Testament -- Jehovah, or Yahweh, is the Hebrew name for God, according to scripture and in the Book of Isaiah, Jehovah calls his followers "witnesses."

The 101-year-old denomination doesn't believe in having a clergy; instead elders run their five meetings a week.

The denomination has made news in the past by refusing to accept blood transfusions, serve in the armed forces, pledge allegiance to the flag or vote in political elections. Members believe that no government can be supported because one day they will be destroyed and God's kingdom will rule.

What they do, however, is spread the name of Jehovah -- something they view as a duty and a way of life. Some like to argue that it was the early Christians who originated the idea of going door-to-door for converts. But it's unlikely they perfected it to such a fine art.

Like crack salesmen, Witnesses assess each household they approach for clues before making their pitches. When Naomi Coffey heard loud rock music coming from one house, for instance, she called on 12-year-old Blake Buckley, who was also working the neighborhood, to make the call instead.

"We find young people would rather talk to other young people than adults," Coffey said. The young people are also used to approach the elderly, since they often enjoy talking to younger people.

When a woman tells Blake's father, Bob, that she is an Episcopalian and not interested, he starts to back off. But when he notices she has a teenage son, he drops a quick reference to drugs and teen-agers and offers her a booklet on the subject. The woman accepts it.

Buckley, a neighbor of Coffey's, said he always identifies himself as a Jehovah's Witness, but Coffey doesn't, because he gets farther along that way. But he said he always wears a name badge that identifies him.

After a string of rejections in the Kettering neighborhood, Coffey decided to try a new approach he had practiced at home with the 7-year-old daughter Danielle.

"Hello, I'm Jim Coffey, a delegate from the convention over at the Capital Centre. My daughter here has something prepared she'd like to read to you. Do you have a minute?"

"I'm sorry, but I'm making blackberry jam and the pots are boiling.

"How about 30 seconds?"

Slam!

At the next house, another housewife answer the door, listens for about 10 seconds then interrupts: "I have three children under the age of four, I'm at a very low patience level, my air conditioning is broken, I have visitors from South America and I don't speak Spanish, and even though I don't agree with everything the Catholic Church says, I'm hopelessly entrenched in Catholicism and I just paid a baby sitter $8 so I could go shopping and I didn't buy anything. Would you like something to drink?"

She invites the Coffeys in. While drinking ice water, they discuss Mormons, "Moonies," birth control, Catholic theology, charity and South America. She does most of the talking. Before wearing out his welcome, Coffey departs, but leaves behind "The Watchtower," the denomination's official publication, and other literature.

Coffey is pleased. He makes a note of the address, which he will pass on to the local district. The woman will receive another, more intense visit within weeks, Coffey says, as will many of the housewives who were "too busy" to talk that day.