At dusk, the old, red brick Amoskeag textile mills that line the Merrimack River in downtown Manchester loom as a ghostly reminder of a bygone industrial era and as a landmark for the city's recent economic revival.

Scores of high-technology companies, banks and accounting firms have opened in the city's old mill yard and in nearby downtown offices. Hotel and office construction is booming, and the area's 5.3 percent unemployment rate is little more than half of the national average.

Many city officials and residents of this heavily Democratic town are descendants of French-Canadian, Greek, Italian and Irish immigrants who worked in the mills at the turn of the century, and they seem convinced that the economy will continue to improve under President Reagan. A Washington Post survey indicates that 68 percent of Manchester residents approve of the way he is doing his job.

"This is a very conservative community," explained Sylvio L. Dupuis, a Democrat, former mayor of Manchester and now president of the Catholic Medical Center. "People like what they see and hear about Reagan . . . . It would take a real change in what's going on, or some candidate who has not yet appeared on the scene, for Manchester to turn away from Reagan."

But serious economic and environmental problems fester here and could sour voters' attitudes toward the president as the political season heats up. Many work for subsistence wages and have difficulty meeting the costs of heating and housing. Water supplies of neighboring towns have been poisoned by toxic wastes, and the Merrimack River is polluted because of inadequate sewage treatment facilities.

Underemployment is a major problem in Manchester, the state's largest city, population 90,936. Industry and local government traditionally have depressed wages--the average yearly wage is $12,328--creating a massive underclass of families that must struggle, even with two salaries.

"We have refined slavery in New Hampshire," said Chris D. Spirou of Manchester, Democratic minority leader of the state House of Representatives. "Everyone works, but for piddling amounts."

For most Manchester residents, many of whose incomes barely exceed the federal poverty level, the high cost of oil, gas and electricity is the most serious problem. Some believe Reagan worsened it by supporting natural gas price deregulation and opposing increases in federal home-heating assistance.

Irene Mullaney, a part-time office manager who lives with her husband in suburban Manchester, complained that Reagan is "a rich man's president" who has done nothing to relieve the financial burden on the average citizen.

"The hardest-pressed areas of this country are not being helped," she said. "I have a big, old house. The cost of keeping a family warm is excruciating. We've gone to burning wood. It's cheaper, but . . . it's a lot of work. What will happen in another 15 years when my husband is no longer able to split the logs?"

The average Manchester household spends $1,700 a year, or about one-tenth of its yearly income, on home heating. Last winter, about 26,000 New Hampshire residents, including 3,200 in Manchester, received federal energy assistance. Twice as many could have qualified, federal officials said.

The Rev. Philip J. Kenney of the Diocese of Manchester said Reagan's cuts in spending for social services and housing have caused widespread suffering here that receives scant attention. He said the number of persons seeking food at his church's soup kitchen, which opened three years ago, has risen steadily.

Last month's average of 109 persons a night was a record.

For years, he added, the arch-conservative Manchester Union Leader, the state's dominant newspaper, has fueled the attitude among public officials and residents that the poor are freeloaders and welfare cheats. Moreover, with no statewide sales or income tax, the level of social services offered by state and local governments has been low.

At the height of the recession last year, the Salvation Army, the lowest rung on Manchester's social-service ladder, received 50,000 requests for aid from individuals, some of them more than once. By comparison, the Manchester Welfare Department, which offers short-term general assistance to distressed persons, handled about 4,800 cases.

City officials acknowledge that many residents are squeezed by rising energy and housing costs, the latter exacerbated by a serious shortage of low- and moderate-income housing. Nevertheless, they insist that most residents manage.

City coordinator John H. Hoben described Manchester as a friendly, unpretentious, hard-working community whose people expect to make it on their own.

Characterizing the city's working class, he said: "Somehow they manage to raise their children well, pay their bills and taxes, not complain and keep their property up. The work ethic is really something."

Residents are also increasingly concerned that authorities are not doing enough about environmental threats, especially illegal toxic waste dumping that has poisoned drinking water in nearby towns.

Discovery of large quantities of potentially harmful asbestos in many public school buildings in 1981 caused an uproar, and the City Council spent $2.8 million to eliminate the hazard.

Now the city is under pressure to dispose of tons of solid waste. Known as "Mount Trashmore," it has piled up in a landfill for eight years since the Environmental Protection Agency ordered the city to close its incinerator.

Residents worry about acid rain, yet the city government did not join other municipalities in adopting acid-rain resolutions.

Much of Manchester's downtown-renewal efforts hinge on reviving the Merrimack River shoreline for recreation and business, but the city dumps about one-third of its raw sewage directly into the river.

Local officials blame the federal government for failing to provide a promised $30 million to $40 million to complete a sewage treatment system. Under EPA pressure, the city built a regional sewerage plant seven years ago but ran out of money before the facility could be connected to the west side of town. Since then, west side homeowners have been charged $120 a year for sewerage services they do not receive.

Fernand Gelinas, a City Council member and Democratic candidate for mayor, said there is widespread dissatisfaction with the EPA for leaving the city in the lurch.

City officials said they have received an equitable amount of federal revenue-sharing and urban development action grants to stimulate private development downtown but have encountered extreme difficulty in obtaining funds for public works projects under Reagan.

The city is having similar difficulty getting federal funds for construction of a replacement for Notre Dame Bridge, a creaky, heavily traveled structure linking downtown Manchester to the west side.

During the 1980 New Hampshire primary, Manchester officials found the Carter White House highly receptive to requests for assistance. Asked whether the city would again seek a White House commitment to fund major projects if Reagan seeks a second term, Hoben smiled and replied, "Oh, yes, yes."