When they heard he was coming, they grabbed a handful of posters and dashed frantically from their storefront headquarters to the front entrance of the Republican National Convention hall three blocks away.

As Ronald Reagan and his wife, Nancy, emerged from their limousine for the surprise midnight appearance last Wednesday at which he would announce his vice presidential choice was George Bush, he spotted them waving their Reagan placards and cheering. He nudged Nancy, then gave them a smile and a wave.

For the members of the Young Americans for Freedom (YAF), youthful shock troops of America's ultra-right, that brief moment epitomized their very special relationship with the man they have been pushing for president for 16 years. Reagan's triumph is YAF's triumph, and his nomination parallels the resurgence of a conservative youth movement that is playing an increasingly visible role in shaping Republican policies and selecting GOP candidates.

"The Republican platform reads like a YAF tract from around 1963," boasts YAF national chairman James Lacy, a Los Angeles attorney. "We feel we've basically got control of the party now and we're not going to let it go."

The YAF presence was felt at this year's GOP convention in a way it has not been felt since the Goldwater debacle of 1964.The group boasted 84 members or alumni as delegates or alternates. It flew or bused in 400 members as part of its $20,000 "Detroit '80 Youth Operation." It also played a major part in the unsuccessful effort to promote Rep. Jack Kemp (R-N.Y.) for vice president.

But when Reagan chose Bush, a man many YAF members dislike and distrust, the group fell quickly into line. On Wednesday night, before Reagan's announcement, YAF executive director Robert Heckman and his wife had joined a YAF contingent in loudly booing Bush when he appeared before the convention to speak.

"You're a loser, George" Heckman bellowed.

But by the next morning, Heckman and other YAF leaders were the first major right-wing group to announce their support for Bush, labeling dissident efforts to nominate an alternative candidate "irresponsible."

"We're not happy about George Bush, but we've worked too long and too hard for Ronald Reagan to let this stop us now," says Lacy. "It just means we'll have to be extra vigilant and work extra hard to make sure Bush isn't Reagan's successor."

"Vigilance" has been YAF's watchword since the group was founded 20 years ago at the Connecticut estate of conservating guru William F. Buckley Jr., YAF members were frontline warriors in the Goldwater movement, then spent the late '60s battling the New Left for control of the nation's campuses. YAF alumni include Maryland Rep. Robert Bauman, conservative mail order magnate Richard Viguerie, and Tom Charles Huston, author of the infamous Nixon-era plan for cracking down on "national security risks."

Right-wing campus eggheads, others fed up with Vietnam-era student strikes and members of what once was branded the Lunatic Fringe -- all have clustered under the YAF umbrella. But the new YAF, which boasts a 20-year high of 80,000 members, 10,000 of them activists, includes a number of soft-spoken, thoughtful people.

"We're not Nazis," says Rod Knoll, a 19-year-old from Arcadia, Calif. "A lot of what we've been saying about the dangers of big government and the need for truly free enterprise are things that people are beginning to believe in."

Virginia, one of the nation's most conservative states, is YAF's home and spiritual base. With headquarters in Sterling, YAF claims more than 3,000 members in Virginia, including 400 at right-wing Baptist Jerry Falwell's Liberty Baptist College in Lynchburg, the single largest YAF campus chapter in the United States.

Falwell's interest in YAF has made the organization's leaders somewhat wary. While they agree with Falwell on "family issues" such as ratifying a constitutional amendment banning abortion, defeating the Equal Rights Amendment and reinstating prayer in public schools, they are concerned that the evangelicals may be improperly blurring the line between church and state.

"I think it's a mistake for any evangelical movement to believe that political philosophy can flow from religious beliefs," says YAF executive director Heckman. "I would hate to have people believe Jerry Falwell's political beliefs are infallible just because they think his religious beliefs are."

YAF has launched many campaigns over the years, ranging from its "Stop Red Trade" effort in 1968, to prevent IBM from selling computers behind the Iron Curtain, to "Operation Expose" in 1973, designed to purge "liberal bias" from college textbooks.

With Reagan launched on his campaign, Jack Kemp is likely to emerge as YAF's new hero for the '80s, the man YAF will turn to keep Bush and other Republican "liberals" from the White House.

But Kemp cannot take Reagan's place in their hearts. Even his selection of Bush has not stopped YAF members from gushing about the former California governor, whose triumph they see as vindication of their philosophy and 16 years of work.

"Now that the world is in so much trouble, people are finally listening," says Charles Cunningham, former Virginia YAF chairman. "They're looking for a new direction and for new hope and that's what he has to offer."