The command post for President Reagan's battle to win $100 million in aid to the Nicaraguan rebels could be found last Saturday at 9:30 a.m. around the conference table in the White House Roosevelt Room.

While Reagan was relaxing at Camp David, the "public diplomacy group" on Nicaragua was meeting -- as it had for the past two Saturdays -- to plot the most aggressive legislative and public relations drive of his second term, one that reaches a climax today with a critical vote in the House.

Insiders say the campaign has been both a carefully orchestrated and surprisingly unpredictable effort by the second-term White House team.

Such minor details as how Reagan would escort special envoy Philip Habib to his car on the day of his departure for Central America were worked out with painstaking attention. Yet such major turning points as a controversial newspaper article by Communications Director Patrick J. Buchanan caught the rest of the White House -- and the president -- off balance.

The stakes for the president and his staff go beyond the contra issue. Since the second term began, Reagan strategists have sought ways to forestall a waning of his power. Last year, on issues ranging from South Africa to trade to the budget, Reagan was thrown on the defensive by Congress. This year, the White House sought to go on the offensive, wielding public relations techniques used so successfully in his first term.

"The White House as a team has been coalescing since Bitburg," said a participant in the Saturday morning meetings. "This is the first time they have really demonstrated the ability to bring all the powers of the White House to bear. It has taken the second-term staff some time to recognize how much power there is in single-issue voters on our side. They are tapping into the network on our side more effectively."

Although there are daily White House strategy sessions, most of the planning for events such as Reagan's Oval Office meeting with rebel leaders and his inspection of captured weapons from Central America began in the Saturday morning meeting.

The informal brainstorming session, over coffee and doughnuts, originated in planning for last year's Geneva summit. The meetings are chaired by Dennis Thomas, deputy to chief of staff Donald T. Regan. The group, which sometimes meets in the Situation Room, also has included Buchanan; legislative liaison Will Ball; political director Mitchell E. Daniels Jr.; deputy national security adviser Donald R. Fortier; Cabinet secretary Alfred H. Kingon; presidential advance chief William Henckel; scheduling director Frederick J. Ryan Jr.; Vice President Bush's deputy chief of staff, Frederick N. Khedouri; and press office deputies Peter H. Roussel or Edward P. Djerejian, among others. Last week, pollster Richard Wirthlin also attended.

After the meeting, an "action memo" is prepared for Regan, outlining plans for the week ahead. On a day-to-day basis, presidential spokesman Larry Speakes also plays a big role.

When the group met March 1, the outlook for contra aid was especially grim. A presentation to some lawmakers by Central Intelligence Agency Director William J. Casey, purporting to show a Sandinista "disinformation" campaign, had backfired with Congress. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had advanced the date of the vote, giving the White House less time.

"We were dead in the water three weeks ago," Buchanan recalled yesterday. "The opposition was broader and deeper than we thought." Buchanan, frustrated last year in his desire for an aggressive campaign on the issue, insisted that this year "we go clearly, boldly and starkly."

Another top Reagan assistant said, "Early on, there was internal disagreement over whether this was an issue you heat up, with television, or an issue where you work the corridors of Congress." The official said national security adviser John M. Poindexter and Buchanan "wanted to heat it up. Those who normally resist stood aside."

"We said to Buchanan, 'Have at it.' "

The first task, they agreed, was raising the level of public attention, putting Reagan on the front lines of the campaign every day to voice his urgent appeal on the evening news and front pages.

The drafting of Habib as Reagan's new special envoy for the region was also part of the game-plan worked out on Saturday mornings. Fresh from the Philippines, Habib's role was to offset charges that Reagan was ignoring diplomacy in his zeal to arm the rebels.

For maximum press exposure, White House officials arranged for Reagan to give Habib a public sendoff from the White House. The two men walked halfway to Habib's car and then stopped and conferred briefly, out of earshot of reporters. This pause was planned in advance, not for any conversation, but rather to create the most interesting photo of the event, officials said.

Although officials planned to "turn up the temperature" on the issue, they did not anticipate how much Buchanan would succeed in doing so with his March 5 opinion article in The Washington Post. Buchanan wrote, "With the vote on contra aid, the Democratic Party will reveal whether it stands with Ronald Reagan and the resistance -- or Daniel Ortega and the communists."

Buchanan bypassed the normal procedures, and did not show what he had written in advance to the president or staff chief Regan, according to high-ranking officials. He did, however, work on it with Oliver L. North, a National Security Council official who has served as a link between the administration and the Nicaraguan rebels.

When the piece was published, to howls of protest from Democrats, White House aides "gulped; they were wide-eyed; they knew there would be a backlash," said one participant in the meetings. "We had asked Pat to lead the charge, but we never discussed how he was going to lead the charge." Afterward, officials simply declared that Buchanan's blast was part of the strategy all along.

Reagan aides also clashed in their private meetings after newspapers carried reports that Reagan was willing to compromise on his proposal. Participants said Poindexter, who was quietly working on possible compromises, rebuked others for making the plans public, complaining that it showed Reagan's hand and could weaken his position.

Although the contra battle nears a conclusion, the Saturday sessions may well go on. At one recent meeting, aides talked about finding new "beyond-the-Beltway" ideas that could be turned into political themes for Reagan. And one frequent lament about the sessions will also go on. In the words of a top White House official: "Why can't we have these meetings on Friday?"