Democrats anticipating big gains in November because of the troubles of Reaganomics shouldn't be complacent, according to a poll of 1,000 voters commissioned by David Garth, the New York political media master who has helped elect a lot of Democrats as well as a Republican or two.

Some things don't seem to have changed much since 1980. Although a majority of those interviewed in mid-June believe Reaganomics favors the wealthy, most also expect that the administration's economic policies will help in the long run and express the belief that the GOP is best able to improve the economy.

Because the favoritism issue hasn't had much impact, the poll, taken by Penn & Schoen, shows that only one-third of the Democrats who voted Republican in 1980 will switch back this year. As a result, Garth concludes that the Democrats are about seven points ahead of the Republicans right now--enough to produce "some shift back to the Democrats in Congress but not enough to produce the rejection of President Reagan and his policies that many Democrats have been privately predicting."

He says Republicans should emphasize Americans' optimism and the long-term benefits of Reaganomics, while Democrats should avoid attacking Reagan personally and hammer on the immediate problems of high unemployment and interest rates.