"I have never been so undecided about anything in my whole life," Pamela J. Miles was telling a carload of convention-bound Democrats as the misty Virginia countryside rolled by. "I'm like an empty cup waiting for somebody to come and pour something in."

For more than four hours, as she piloted her station wagon from Fairfax County to Roanoke, Miles' friends eagerly extolled the virtues of county prosecutor Robert Horan and former congressman Joseph L. Fisher--their favorite candidates for the party's U.S. Senate nomination.

But within half an hour of arriving at Roanoke's cavernous convention center Friday evening, Miles, 41--an aide to Fairfax County Supervisor Audrey Moore--and her friends knew that the persuasion had been all but pointless. The word was out on the convention floor: Lt. Gov. Richard J. Davis was to be the nominee.

"I guess it really didn't matter who I voted for," Miles grumbled later, mildly miffed that the real work of the convention was done in private meetings far from the open caucuses she attended. "It was all decided before we got here."

In the end, it wasn't the candidates' positions on issues that determined Miles' vote--"They're really not that much different," she said. Nor was it their organizations, or even their electability in a possible three-way race against Republican Paul S. Trible and Independent Sen. Harry F. Byrd Jr. in November.

What Miles listened to most, amid a two-day maelstrom of campaign posters, balloons, stump speeches and stale hot dogs, were the urgings of her Annandale friends and bridge partners.

People such as fellow car-pooler Norma Heck, and Val and Fran McWhorter, who are almost as close as members of her own family, persuaded Miles to make Horan her first-ballot choice--more out of friendship than anything else. "If I didn't vote for Horan, it would really disappoint Val and Fran and Norma," she said. "And since it didn't seem to make any difference, why not?"

As chairman of the Annandale district committee, Miles will be stumping for Davis this year. But as she watched the flood of conventioneers start for home Saturday afternoon, she wondered aloud whether the closed-door process will inspire much enthusiasm for Davis' candidacy among her Northern Virginia troops.

"There really wasn't much emotion at the end. Nobody was really upset and nobody was particularly overjoyed, either," Miles said, cradling a hat with a green "Horan" sticker on her lap. "That could prove to be a problem."

As the windshield wipers slapped against a pesky rain, Pam Miles reached back 20 years to talk about her baptism in politics--back to Coon Rapids, Minn., where she helped paint a deserted gas station gold for Barry Goldwater.

It wasn't a very auspicious beginning for a devoted Virginia Democrat, Miles conceded amid gales of raucous laughter from the passengers in her Ford station wagon. "Everybody's got to start somewhere," she said, adding that the paint job was her husband's idea.

Yet somehow it gave her an interest that survived Watergate ("I felt like someone had personally burglarized my house," she said) and transferred from the Republicans to the Democrats when Jimmy Carter ran for president.

In the years after Carter's 1976 campaign, Pam Miles became the quintessential Democratic campaign worker. Hanging in the basement of her brick split-level are autographed sketches she's done of the candidates she's worked for: Carter, Gov. Charles S. Robb, former Virginia attorney general Andrew P. Miller, and current state Attorney General Gerald Baliles, her Democratic hero, whom she served as a campaign coordinator last year.

As it turned out, it was Baliles who gave Miles her first clue that the deal had been struck for Davis. Standing in the lobby of Roanoke's Patrick Henry Hotel, Baliles was uncharacteristically quiet when Miles asked for his predictions. "Two ballots at the most," he said with a mysterious half smile.

Arriving at the convention hall Friday evening, Miles and her friends found the atmosphere strangely subdued. The recent fracas over Del. Owen B. Pickett of Virginia Beach, who was the party leaders' anointed candidate early this spring only to be rejected last month by the rank and file, had left candidates little time to organize.

There were only a handful of the brightly colored handbills, stickers, hats and signs that usually litter political convention halls. Even the usual ear-splitting din had been toned down. If the half-dozen favorite son candidates were trying to lobby undecided delegates, they were certainly being quiet about it.

By the time Miles heard the national anthem signal the start of the convention Friday night, she was mildly aggravated. Aides of Davis and Robb were circulating among the clumps of delegates on the convention floor, putting out the word that the governor supported Davis' candidacy and urging Horan, Fisher, state Sen. Virgil Goode and Senate Majority Leader Hunter B. Andrews to step aside.

Orange "Draft Davis" buttons and signs began to sprout everywhere, leaving many to complain that the "draft" was really a well-orchestrated campaign by Davis, who had formally pulled his name out of consideration only two weeks before.

Even though she had no objection to Davis himself, Miles felt the party rank and file had a right to choose its own Senate candidate. "We've been maneuvered once and we don't want to be maneuvered twice," she said, recalling Pickett's ill-fated candidacy as she waited in line to buy a soft drink and hot dog. "I just hope it's not all over before it has even begun."

Back in the Fairfax delegation, Horan's backers were on the warpath. "I can't believe this," groused Norma Heck, who worried that a vacancy in the lieutenant governor's slot could give Republicans a springboard to the governor's seat. "We work for 16 years to get ourselves a governor, and now we're ready to tear it apart our own stupid selves."

But their anger was soon to be defused by more than a dozen hospitality suites and parties and--of all things--a rip-roaring speech on military preparedness from the keynote speaker, retired Lt. Gen. Samuel V. Wilson. After criticizing the "yachts" of Reagan's Republicans, Wilson called on Democrats to "swim past those beached boats and spawn."

Came a voice from the Fairfax crowd: "I'm too young to spawn."

Came another: "I've done it too many times already."

And a third: "I don't even know what spawning means." The group's convulsive giggles erupted repeatedly throughout Wilson's message.

By the next morning, when they assembled bleary-eyed to hear speeches by favorite sons Horan and Fisher, members of the Fairfax delegation were not hoping for anything more than two or three ballots at most. Most of them were resigned to a Davis nomination, having learned that the lieutenant governor had formally declared himself a candidate at 11 the night before. At least two ballots would allow Horan and Fisher to save face, their backers reasoned.

Nor were they hopeful for the fate of a Fairfax-backed proposal to oust House Speaker A.L. Philpott from the convention podium over his opposition to the Equal Rights Amendment. Miles, looking wide awake despite a party schedule that lasted until 4 a.m., sat in the back row of a 9 a.m. women's caucus meeting to hear speakers urge conventioneers to rally around the ERA.

"This is a real effort in futility," she said, voting for the anti-Philpott proposal anyway. "We can sit here all day and it's not going to do any good." The proposal squeaked out of the caucus on a narrow vote, but died on the floor of the convention.

Three colas, one hamburger and several handfuls of popcorn later, Miles was sporting a styrofoam boater with a bright green "Horan" sticker wrapped around the crown. It stayed on her head throughout the first ballot, when her vote was among 166 Horan received. Davis, by comparison, weighed in with 1,369 delegate votes--only 175 short of the 1,544-vote majority he needed to win.

Sitting in the balcony, looking down on the swirling convention floor, Pam Miles said she was not really sorry about the way things were turning out. Jerry Baliles was coming out ahead. If Davis won the Senate seat and his lieutenant governor post were left vacant, she reasoned, Baliles could easily be the Democratic nominee for governor in 1985.

Horan gave a conciliatory speech , and his Fairfax backers--including Miles--switched to support Fisher on the second and final ballot.

"There isn't anything else you can do that's quite as dramatic and as emotional as this," Miles said. "Look at me--I'm teary-eyed. We're just like a family. You work together, you fight with one another, you cry, you make up."

In a matter of moments, the announcement crackled over the loudspeaker system: Davis had swept the convention with 2,009 delegate votes. The convention was over. It was time to head for home.