Rarely has a president had such an opportunity to blend his main policy concern with a traditional speech about charity.

Since 1927, American presidents have been urging citizens to support United Way charity drives, but none has had as pithy a message to deliver as President Reagan did this week.

"Our nation is entering a period of transition," Reagan said on behalf of United Way just three days after his fifth and most recent national television address stressing reductions in federal programs. "And I'm counting on that voluntary strength and that commitment as we turn from government doing for us that which we can best do for ourselves."

In case some Americans have not been paying attention during the first eight months of his administration, the president pointed out that somebody will have to pick up the slack when government pulls back.

"We are shifting responsibility away from the federal government, but to do so requires people's needs to be met at the local level. We need new, imaginative partnerships between state and local governments and voluntary organizations if people's needs are to be met," Reagan said.

"I can think of no better place to begin that in this fall's United Way campaign," Reagan said.

Officials of the charity organization are delighted with Reagan's speech, which was taped in the White house and broadcast as the kickoff of the giving campaign. "We feel it's the strongest message we've gotten from the White House," spokesman Steve Delfin said.

The tradition of doing a kick-off broadcast, begun by one of Reagan's favorite presidents, Calvin Coolidge, on radio in 1927, gave Reagan an opportunity for a slightly different approach to a message he has been delivering with increasing frequency:

Private citizens' generosity will do what government used to do and will do it better.

"As a people we have a proud tradition of generosity," Reagan told the nation in his message on the budget last Thursday evening. "Our recent emphasis on volunteerism -- the mobilization of private groups to deal with our social ills -- is designed to foster this spirit of individual generosity and our sense of communal values," Reagan said in a speech Monday about crime.

Speaking for the United Way, he could be even more direct: "I ask all individuals of every income level and I ask all corporations of every size, to give more generously than ever before to this fall's United Way campaigns in your communities."

Other presidents have made similar pitches, but they have not seen things quite Reagan's way. A glance through the texts of their remarks recalls different times and different attitudes.

"This is a time of great change in our country, the economic, social and scientific life," President Kennedy said in 1963.

In 1968, President Johnson was launching the Great Society and spoke warmly of "government, business, labor, minority groups and others" joining in alliances to attack urban ills. He urged that the voluntary agencies not be neglected.

A year later, President Nixon made a Reaganesque point when he listed the numbers of volunteers and agencies who work with United Way. "When I saw all these facts I thought, 'Well, this sounds like government,'" Nixon said.

Reagan would have it act as government has been acting. If all goes well, people might turn to him and say: "Thanks to you, it's working."