Although President Reagan won a smashing congressional victory on his federal budget cuts, he is paying a price in the rising number of Americans who believe that his economic program will be unfair to the elderly, those on pensions and the poor.

The latest Harris Survey has revealed some trends that could spell trouble for the president and the Republicans in next year's elections:

By 61 to 33 percent, a majority of Americans believes that Reagan's new economic policies "will be unfair and cause hardship on the elderly and those on pensions." In February, just after the president announced his massive spending and tax cut proposals, a 56-to-38-percent majority thought his economic plan would be fair and equitable for older citizens and those on fixed incomes. By May, the assesment of his plan was reversed and a 53-to-38-percent majority sensed inequities. Now, 61 percent think the new economic policies will cause hardship for the elderly, up 23 points in four months.

But 57 to 34 percent, a majority now believes that the president's economic policies will be "unfair to the poor and will cause hardship on them." In February, a 49-to-45-percent plurality thought the poor would not be hurt by the Reagan program. By May, this had turned around to a point where a 50-to-41-percent plurality thought that there would be hardship for the poor.

By 54 to 39 percent, a majority still feels that the president's economic plan will be fair and equitable for them. However, back in February, a much higher 70-to-25-percent majority felt that way, so there's been a decline of 16 points over the past four months.

The groups that are worried most are people in the East and Midwest, big-city dwellers and rural residents, people under 30 and over 65, women, blacks, people with incomes under $15,000 a year, skilled labor and white-collar workers, union members, Democrats and liberals.

The Reagan program is thought to be "fair and equitable" by larger numbers of people in the South and West, suburban and small-town residents, people between 30 and 49 years of age, men, whites, those in the higher income brackets, businessmen and conservatives.