When Ronald Reagan goes before Congress this week to hawk the glories of his new economic policy, he will not be depending solely on his own formidable communicative powers to make the sale.

Moving in tandem with the president will be an "educational campaign" designed to convince the American public, and through this public the Congress, that Reagan's broadscale budget-cutting plan should be approved quickly and without changes in the interest of rescuing the nation from what the president has called "the worst economic mess since the Depression."

The plan has many parts. It includes a volunteer committee outside the White House, reminiscent of the Citizens for a New Prosperity which once tried to sell the Nixon program of wage-price controls. It will include fact sheets mailed to influential trade and issue groups, and high-powered speakers who will appear on selected forums throughout the nation. And it will include a carefully coordinated series of appearances on major television programs by the designated economic hitters of the administration -- Office of Management and Budget Director David A. Stockman, Treasury Secretary Donald T. Regan and Council of Economic Advisers Chairman Murray Weidenbaum.

"What we're after is a communication that the president's program is needed, desired and irresistible," said a White House aide last week.

Known now simply as "the coalition," the volunteer committee leading the charge will be headed by Charles Wick, a millionaire Hollywood investor who is a longtime personal friend of Reagan and in line to become director of the International Communications Agency. Wick will practice up for his prospective future job of selling the world on Reagan policies by attempting to convince his fellow Americans of the benefits of vastly slashing the federal budget. He will have experienced assistance from Stuart Spencer, Reagan's election campaign adviser, who will coordinate the attempt to line up prominent Americans and organizations.

The front-line work of this committee will be performed by the veteran Washington public relations firm of Wagner & Baroody, which has served three Republican presidential administrations and a wide variety of GOP candidates.

And the White House is coordinating next week's approach through a loosely formed communications group headed by former Ford director of communications David Gergen and prominently involving the White House press office and political adviser Lyn Nofziger.

To make sure that the proposals Reagan will make in his speech to the joint session of Congress receive the desired official interpretation, the White House is withholding its economic spokesmen from television today and making them available en masse starting Thursday morning, the day after the president's speech.

Stockman, the youthful matinee idol of the Reagan troupe, will do his number on NBC the morning after the speech and on the CBS show "Face the Nation" the following Sunday.Secretary Regan will reverse the roles, appearing on CBS the morning after and on NBC's "Meet the Press" on Sunday.

And the White House will do its own part in the educational campaign, bringing in editors, columnists and assorted opinion-makers to breakfast and lunch with these economic spokesmen as well as briefing reporters 10 hours before the speech.

The line for the meetings already has been unofficially laid down in the terminology used within the White House to describe Wednesday's speech. It is called the "road-we-must-take speech" to distinguish it from Reagan's Feb. 5 message -- the "road-we're-on speech."

Beyond the immediate flurry of self-induced publicity, the White House will rely on the volunteer committee and Wagner & Baroody.

"We'll be acting as a catalyst for a number of organizations that we expect will support the president's program," said Joseph Baroody.

Among these groups, he said, are trade associations, labor unions, farm groups and some single-issue organizations.

The public relations firm will provide fact sheets and other material aimed at convincing Americans that the United States can be rescued from pending economic disaster only if citizens support the Reagan budget cuts in full. One selling point will be the contention that the president's program is a balanced one which must be kept intact so that "special interests" do not pit one provision against another and thereby destroy the entire package.

"We have to talk in terms of the national interest as a whole and what's good for the country," said a well-placed White House official.

He contended that this national appeal came fairly easy to a Republican administration and said Reagan could be in trouble if the economic cuts were considered in terms of interest-group politics, "where the Democrats are much more successful than we are."

While there was much talk after last November's election that the ultimate weapon of a Reagan administration in behalf of its economic program would be the veiled threat of retaliation at the polls against recalcitrant Democrats, that is not the thrust of the present public relations campaign.

"There are a number of Democratic congressmen who would like to vote for this program if they feel they can get away with it back home," says a White House official.

The administration hopes to make it easier for these Democrats by selling the program as one that takes from the rich as well as the poor in the interest of the entire country.

"We're not zeroing in on members of the Congress at this point," says Baroody, echoing this view. "We're trying to create a favorable climate of media coverage and popular approval. And we want to resist breaking the program apart."

Among the instruments of persuasion will be a bureau of what Baroody calls "sophisticated speakers" who will take the administration's case to the nation. No one has been officially designated for this role as yet but persons such as former treasury secretaries John B. Connally and William Simon are the kind of spokesmen the committee has in mind.

The campaign for the new economic policy is unlike the "Whip Inflation Now" program in the Ford administration, which was run entirely out of the White House. But it is highly reminiscent of the Nixon administration campaign for wage and price controls.

An important difference, says Baroody, is that Nixon was struggling to regain diminished popularity with a less-than-popular program while Reagan currently is riding the high tide of public approval at a time of widespread support for budget and tax reductions. There is another difference which may be even more important. Reagan is a far better salesman of his own product than any president since John F. Kennedy and the promoters of the "educational campaign" know they have him waiting in the wings to overcome latent buyer resistance.

"The major support for marshaling further support for the president's program lies directly with him -- both in convincing a broad spectrum of people that his program will work and in convincing the Congress," says Reagan pollster Richard B. Wirthlin, who is taking a national poll which includes questions about the economic program.

This is the view within the White House as well, where Reagan is considered "The Great Communicator," capable of overcoming the interest-group opposition the economic program is certain to generate.

"The president is the single most important tool we have," says a White House official.

But in the selling of the president's program. Reagan will not operate alone.