Landlords, beware: Tenant activism is on the rise.

In the past year or so, apartment dwellers have pushed through rent control laws in places as disparate as Allenstown, N.H., Elizabeth, N.J., and San Francisco. They've won a plethora of battles to get city and state laws that restrict condominium conversions and protect tenants against eviction. And although landlords are fighting back and winning some battles, many observers believe the trend is irreversible.

"In the old days, dissatisfied renters would vote with their feet -- they'd move somewhere else," says Louis Masotti, professor or urban affairs at Northwestern University. "How they're fighting for their turf." Adds Randall Pozdena, an economist with the Federal Reserve Bank of San Francisco: "The rhetoric has turned sharply antilandlord. The battle is escalating."

It's easy to see why. The National Association of Home Builders forecasts that about 316,000 rental units will be built this year, down from about 396,000 last year and 469,000 in 1978. The units going up this year, the group says, aren't even enough to replace those abandoned, destroyed or converted to condominiums.

Yet demand for apartments is growing. As home prices solar, more people are forced to abandon dreams of ownership. Furthermore, many young, childless couples who prefer city life are eager to rent. The results: ya national apartment-vacancy rate of only 5 percent with substantially lower rates in Washington and such cities as New York, Chicago and Seattle, and rapidly rising rents.

If renters are unhappy, they often don't have the option of looking elsewhere. Says Richard Francis of the National Rental Housing Council, "It's all part of the consumer movement. People aren't willing to accept a landlord's statements anymore, and there's a growing feeling that "this is my place, my home, whether I rent or own."

Landlords contend that rent increases and condominium conversions are natural responses to the economics of apartment owning. They say restrictions will accelerate the destruction of rental units and further reduce the already slow pace of construction.

But tenants disagree, and they're banding together to fight. Until recently, tenant activism focused almost entirely in New York City and a few places in California. But now cities all over the country have tenant groups. There's even a National Tenants Union, formed earlier this summer by 80 local organizations.

The willingness to fight starts at the bottom, where observers agree there's plenty of support. In Arlington, for instance, 170 residents of one building in a fashionable section joined to protest faulty air conditioning and dirty grounds. "If we're going to pay this much, we want, and expect, certain things," says Rebecca Cox, whose two-bedroom apartment rents for $500. Cox said the group requesting rebates and was ready to go to court.

At the local and state levels, politicians often go along with the tenants' demands. That's partly because there are more tenants than landlords, but some observers see a more subtle reason.

"In the course of inflation, a lot of the more well-to-do tenants have become owners, almost in self-defense," says John Weicher, director of the housing markets program at the Urban Institute. "So you've got a lower-income group being tenants, which gives them a better political hearing because they are more clearly defined."

In Seattle, for instance, where 30 percent of the population rents, the city council recently passed a law that makes it more difficult for landlords to evict tenants. Tenants also expect to have an initiative on the November ballot that, among other things, would limit rent increases and apartment demolitions. "A year ago, when we first raised these issues, everybody said this couldn't be done," says Judy Kuskin, a local tenant activist.

But Seattle has a vacancy rate of 2.1 percent, rents have risen 60 percent in the past three years and condominium conversions have been rampant. "The longer the vacancy rate remains low, the higher the consciousness of the public toward the plight of the renter," says Dewey Potter, an assistant to the city councilman sponsoring the bill.

Consciousness has been raised to a particularly high level for condominium conversions. In the past year, dozens of cities, including Washington, Evanston, Ill., and Eugene, Ore., have imposed moratoriums on conversions. Legislators in West Virginia and Pennsylvania have passed laws that protect tenants from indiscriminate conversions.

"There's no question that the free and easy days of developers going in and converting buildings on their own terms is over, at least in the North and Eastern cities," says Robert Wittle, a real estate lawyer in Washington.

Meanwhile, landlords are trying to pull together to stem the tide. They are challenging some of the laws in court, and they have overturned condominium-conversion moratoriums in Chicago, Verona, N.J., and elsewhere. "Developers understand that if they don't parlay their collective strength," says Northwestern's Masotti, "they may very well be moratoriumized out of business."