Most people, when they think of the Salvation Army, think of coffee and doughnuts, bells on the corner at Christmas, a bed, a roof, a helping hand.

Mayors of large cities are particularly partial to the army. When people are being driven from their homes by water, the Salvation Army does not ask whether it was caused by a broken water main or a cloudburst. They just pile in with the coffee and the blankets and the cots.

Generally speaking, no organization has a brighter name or a warmer place in the hearts of the down-and-out or the down-on-their-luck.

But when the federal government thinks about the army, its brow wrinkles. It is gripped with the fear that the Christian soldiers may be actually conducting an assault on the Constitution.

The Department of Housing and Urban Development is especially vigilant in this regard. One official, turning down a request for a Community Development Block Grant for a homeless shelter in Reading, Pa., wrote:

"The shelter (albeit with a secular purpose) is owned and operated by the Salvation Army, a pervasively sectarian organization."

"Pervasively sectarian" is quite an indictment. It sounds like a particularly bad fungus growth or a disease for which there is no cure.

" T he service is not provided in a truly religiously neutral atmosphere," wrote Joyce Gaskins, program manager of the HUD Philadelphia regional office in a May 30 letter to Reading, Pa., city officials, as she denied money to "this worthwhile activity."

The Housing and Urban Development Department is proceeding along this curious path in conformity with a previous ruling of its own, enunciated by Associate General Counsel Robert S. Kenison. In April, 1985, he wrote a letter denying a block grant to the Salvation Army-run Share House, a half-way house for drug addicts and alcoholics in St. Joseph, Mo.

Kenison had discovered that Share House personnel encouraged its clientele to "seek spiritual nurture."

For that sin, Share House was turned away, and the Salvation Army closed it down. No one charged that the Salvation Army was proselytizing for its own church, or that it was mean to patients who spurned "spiritual nurture." But HUD seems to be a little Supreme Court, with the Constitution more consulted than the building manual.

It makes you wonder if the HUD folk have been listening to their leader, whose concern with separation of church and state is not exactly an obsession.

President Reagan repeatedly, eagerly, bounds across the line. He pleads for prayer in public schools, presses for tuition tax credits for parochial schools. He is a Bible thumper. In a 1983 speech, he said, "Inside its pages lie all the answers to all the problems man has ever known."

And who has been more of an evangelist for partnership between church and state in solving the problems of the poor? He has endlessly exhorted the private sector to work with the government to help the unfortunate as as he cuts federal funds for that purpose.

The zealous separationists at HUD are marching to a different drummer. They have already denied funds to five projects in other states -- unless the Salvation Army sets up secular corporations to monitor any constitutional violations.

The cost of such "antiseptic corporations," said Salvation Army attorney William Moss, would be prohibitive for all their undertakings, and "absurd" when the HUD contribution is something like $5,000.

Moss suggested that a contract between the organization and the government could cover all the questions that bedevil HUD's hairsplitters. Moss' made the suggestion at a Capitol Hill fact-finding session, held under the auspices of Sen. Slade Gorton (R-Wash.).

HUD counsel Kenison said somewhat languidly that the guidelines are under review at the Justice Department.

Meanwhile, representatives of other religious charities testified to "the incredibly chilling effect" the nationwide application of the St. Joseph precedent is having on projects undertaken at the begging of state and local officials.

It is not the first time the Reagan administration, while preaching cooperation, has tried to see that the poor don't get too much. Hardly had the president been sworn in when the administration moved to block a plan to distribute to four major charitable groups -- including the Salvation Army -- $1 million in ill-gotten oil company gains, money companies had paid the government for overcharging consumers. After the publicity, the administration cried uncle and gave out the money.

Possibly, HUD will see once again how blockheaded it is to go to war against the Salvation Army, the army that even doves love.