Just before signing a job training bill last week, President Reagan appeared in the White House Cabinet Room with 20 youthful job trainees brought in to witness the ceremony. He told them: "You are the focal point in the main problem today, unemployment . . . . "

They also were part of the solution last week as Reagan used the White House as a stage from which to campaign for Republican candidates and his economic program in the final weeks before the Nov. 2 midterm election.

Reagan never went more than a few blocks from the Oval Office, but he carried out a campaign blitz that White House officials think is just as important and perhaps more powerful this year than his appearances on the hustings.

This is because Reagan has always been more comfortable and successful as a mass communicator than as a political stump speaker.

His career was built on the rise of radio and television, and today they remain his most potent tools in a campaign coming directly from the Oval Office.

"You just can't separate governing from politics three weeks before an election," a senior administration official said.

Nor did the White House attempt to separate them last week as Reagan engineered a radio and television blitz.

He signed the jobs bill to demonstrate his conviction to do something about rising unemployment.

He went on television to cast his economic record in terms of "important progress" in "four out of five" major problems.

He went on radio yesterday to reiterate the message and on Friday to tell midwestern farmers of his efforts to sell their overflowing grain stocks to the Soviet Union.

He also went to the Justice Department to announce a major crackdown on drugs and organized crime and appeared on a satellite telecast to rally GOP faithful for the final weeks of the campaign.

Behind all this activity lies the realization among White House officials that with the nation locked in recession, and some areas plunged into depression, Reagan can be of only limited value in personal appearances for Republican candidates.

There are some places where he cannot and will not campaign.

But Reagan's advisers feel he can "raise the level of the lake" for all GOP candidates by campaigning against the backdrop of the executive mansion.

"There are a lot of things he can do without ever leaving the White House complex," said a Reagan adviser who recalled how President Ford effectively employed the Rose Garden strategy against Reagan in 1976.

Reagan has not carried it to extremes.

He has traveled this fall and will make two or three more campaign forays before the election, beginning with this week's two-day swing through Nebraska and Illinois.

But, in a string of events last week, he demonstrated how to stay above the fray and yet be part of it.

For example, Reagan's radio speech Friday was broadcast across the Farm Belt, where Republicans are struggling against a growing mood of despair as grain elevators are padlocked and Main Street stores boarded up.

The president sounded a note of sympathy. It was, he said, a "fact of life in America's heartland . . . that things haven't been very good down on the farm."

Reagan went on to announce that he was offering the Soviets some of this year's bumper crop and that he would guarantee such purchases against disruption if made during November and completed within 180 days.

Although the United States had lost a large share of the Soviet market, he assured the farmers, "We're on our way back up."

"By doing that farm speech," a White House official said later, "he helps dozens and dozens of Republicans" in the troubled Plains states and Midwest.

The president's televised address on the economy Wednesday night offered another example.

Although it was scrubbed of any partisan reference to Democrats, and White House officials insisted that it would be nonpartisan, the speech laid the groundwork for Reagan's campaign theme: that his policies have attacked the nation's economic ills instead of aggravating them.

Indeed, White House officials began the week determined, as one put it, to "blunt" the political impact of the 10.1 percent unemployment rate announced the previous Friday. The speech was part of that effort.

"Our view is that we could honestly say it was nonpartisan," a senior administration official said. "We never said it was nonpolitical."

Reagan's advisers have long agreed that his strength lies in using the airwaves, and that figures prominently in future plans.

The president is expected to make a paid television appeal for Republican candidates on election eve.

But Reagan's campaigning from the White House left some administration officials wondering last week where to draw the line.

For example, although there had been talk about establishing crime-fighting task forces earlier, the announcement Thursday by Reagan of 12 task forces and 1,200 new investigators and prosecutors was apparently planned so quickly that many top law enforcement officials had to rework their schedules at the last minute so they could attend the president's speech.

Some confided afterward that they did not know whether the program would work or was simply put together with election year "mirrors," as one put it.