When President Reagan meets the press tonight at his 12th formal news conference, he will be departing briefly from a carefully considered White House strategy of keeping out of public controversy during the dog days of summer.

Since returning to Washington two weeks ago from the first half of a secluded summer vacation at his ranch in California, Reagan has seldom addressed, and never in detail, such key problems facing the nation as the limping economy and the tense, deterioriating situation in the Middle East.

Instead, he has gone out to St. Louis to talk baseball at a youth club, up to Capitol Hill to lead the cheers at a White House-orchestrated rally for a balanced budget amendment, and stood in the Rose Garden at a gathering of staunchly anti-communist eastern European ethnic groups to denounce the Soviet Union and, in sonorous baritone, join them in singing "God Bless America."

It has been government by symbolism, an emphasis on the atmospherics of the presidency that is part of a deliberate strategy, according to White House aides.

They regard this summer as a grace period for Reagan before the fall congressional election campaign. They say they have sought to avoid presidential mistakes on politically sensitive issues before Reagan goes back to California in mid-August for the second half of his vacation.

When he returns to Washington shortly before Labor Day and the campaign heats up, these aides expect Reagan's actions to be controlled by events. So, in these languid days of a sweltering summer, they are taking advantage of what may be their last opportunity until November to control events themselves.

Reagan has used the time to raise the banner for his New Federalism initiative and his foreign aid and investment program for Caribbean Basin countries, both pet causes that got lost in the budget battles on Capitol Hill last winter and spring.

But politics, and Reagan's slide in opinion polls since his triumphant victories in Congress only a year ago, are never far from the minds of his senior advisers.

His stage-managed appearances in St. Louis and on Capitol Hill, under the close direction Michael K. Deaver, his deputy chief of staff, were designed to counter any perception that Reagan is uncaring about the problems of the poor or that his record budget deficits in his pending budget signal a retreat from his longstanding goal of balancing the budget.

Meanwhile, there have been a number of planning sessions for the campaign. Pollsters Richard Wirthlin and Robert Teeter have been in and out of the White House with raw data from surveys they have been taking for Reagan. His chief political adviser from the 1980 presidential campaign, Stuart Spencer, also has been around to help advise on how to handle this year's campaigns.

Reagan's advisers are still groping for a strategy for using him in the fall campaign, although one senior adviser said there is a "solid consensus" against sending him out on the campaign trail, "shirttail flapping."

"People want their president to be president," this adviser said. "We don't want to turn a first-class president into a political huckster out selling candidates."

But the classic Rose Garden strategy may not be the answer for Reagan, either. There is a feeling among his advisers that he must travel the country and be seen among the people to improve his sagging image.

While surrogates from his Cabinet and Capitol Hill are expected to bear the brunt of returning the Democrats' volleys, there is also a belief that Reagan cannot continue to avoid defending his economic program.

Since their return from California earlier this month, White House aides have attempted to smooth the waters for him, going to elaborate lengths to back away from the administration's previous predictions that the country would be in the midst of a robust economy recovery by now and to caution that it will take longer to occur.

They are experimenting with catchy slogans to describe the current state of the economy in politically advantageous terms. A White House spokesman last week referred to the current period as a "turning zone." A day later, presidential communications director David R. Gergen, attempting to shrug off questions of whether the poor condition of the economy would hurt Republicans at the polls, said Wirthlin had discerned a "patience factor" among the public in his polling.

There still appears to be strong resolution in the administration, however, that there should be no change in the economic program before the November election. Aides said this is due in large part to Reagan's strong conviction that it will work eventually and to the belief that switching signals now would give the appearance of the "flip-flopping" they believe damaged former President Carter.

But maneuvering a path between consistency and the demands of politics has not always proved easy for Reagan, either. An example of this has been the debate over grain sales to the Soviet Union, a topic Reagan is certain to be asked about at his press conference tonight.

A consensus has emerged that the administration should extend for another year the existing agreement with the Soviets which expires Sept. 30. But Reagan's principal advisers still disagreed yesterday over the amount of grain the Soviets should be obligated to purchase.

Advisers mindful of the political impact on American farmers seeking to sell surplus grain were urging Reagan to raise purchase levels. Foreign policy hard-liners were reportedly arguing against that, concerned that any increase would undercut efforts to pressure the Soviets to ease martial law in Poland.

There was some talk of deferring a decision until late August when a new agreement would be signed.

Carter had conducted 35 press conferences by this point in his administration, nearly three times the number Reagan has had.

Reagan advisers say they are pleased with the way he has handled questions at his recent prime time news conferences, a distinct contrast from the way he often stumbled over questions earlier.

These news conferences have followed intensive preparation and there are indications, although senior aides deny it, that Reagan still does not follow closely the details of events.

When he talked to reporters on Air Force One while returning from California earlier this month, Reagan did not exhibit a firm grasp of issues involved in the Middle East, although aides had attempted to depict him beforehand as intimately involved in the negotiations over West Beirut.

Earlier, when Reagan had met at the White House with Israeli Prime Minister Menachem Begin, the Israeli leader was surprised when Reagan produced a set of index cards from which he read to state the U.S. position.