IF THE LANDLORD of a large Massachusetts apartment building decides to sell it off, the tenants inside have few options other than hope the next owner doesn’t have plans to price them out. Occasionally a local community organization or city partnership will manage to step in to buy and preserve the building as affordable units.

More often, the waves of the market wash the housing out of reach for longtime, lower-income tenants.

Housing advocates say working to build new homes, including subsidized affordable housing, is essential, but can’t make up for the loss of existing lower-cost units. “We like to say trying to solve the housing crisis without preserving affordable housing is like trying to fill up a bathtub with the drain open,” said Ann Jochnick, a housing attorney with the Massachusetts Law Reform Institute.

The social justice advocacy group is one of the organizations backing legislation that advocates say could go a long way toward preserving existing lower-cost housing by giving tenants a chance to band together and buy a building being put up for sale.

The House and Senate each tucked a version of the Tenant Opportunity to Purchase Act, or TOPA, into its economic development bond bill this summer. A six-member conference committee is now negotiating a single version of the omnibus bill to send to Gov. Maura Healey.

Broadly, the policy would let renters in a multifamily building being put on the market organize to match an outside buyer’s offer and purchase the property themselves, or hand that right to a nonprofit or community land trust.

Right to purchase laws for tenants first arose in the late 1970s, with one of the strongest measures now on the books in Washington, DC, which adopted the policy in 1980.

Advocates credit it with preserving more than 16,000 units of affordable housing in DC, and a government study found that housing production continued apace, but real estate groups say it needlessly extended the period of time it took to sell buildings and drove away third-party bidders. They argue that recent changes to DC’s policy were necessary because the tenant right-to-purchase rules were slowing down the sale process and putting landlords in a bind.

Sen Pat. Jehlen, a Democrat from Somerville, who put forward tenant right-to-purchase legislation in Massachusetts, said it “would allow communities to slow displacement and gentrification by removing some homes from the for-profit, speculative, escalating market, and making them permanently affordable.”

Legislative opponents, however, say the policy will harm the housing market.

Rep. David DeCoste, a Norwell Republican, said he is concerned that the measure “will have a severely detrimental impact on small property owners.” He said it will “diminish the value of existing properties and discourage potential investors from buying existing properties or building new multifamily homes.”

State Sen. Lydia Edwards, a Democrat from East Boston, who is one of the measure’s Senate co-sponsors, points out that tenant right-to-purchase laws are nothing new in Massachusetts.

Since the 1990s, residents in manufactured housing like trailer parks have had the right of first refusal to buy the land if it will be sold, and a law has been on the books since 1983 requiring most landlords who plan to convert apartments into condominiums to give the tenants the right of first refusal to buy their unit.

“We’ve always had an option to purchase,” Edwards said, “but we’ve just narrowed it to specific kinds.” The new forms of legislation target tenants in large rental buildings, she said, “because they’re being displaced.”

Advocates say the types of buildings that TOPA tends to cover — unsubsidized low-cost units known as naturally occurring affordable housing — are much cheaper to preserve when compared with the cost of building new subsidized affordable housing. And the units, which make up almost half of the affordable housing in the state, are dwindling and often in the sightline of investors.

The Legislature passed a tenant opportunity to purchase measure at the very end of its 2019-2020 session, as part of an economic development bill. Then-Gov. Charlie Baker signed the rest of the roughly $627 million package, but line-item vetoed the tenant right-of-first-refusal section, saying he was concerned “that making multifamily sales transactions less predictable will result in less investment and construction of fewer rental units.”

This session, the policy came back as standalone legislation in both chambers, put forward in the House by Democratic Reps. Jay Livingstone and Rob Consalvo and by Jehlen in the Senate.

Cassie White, director of policy and community organizing for the community development organization Fenway Forward, testified in favor of the on the standalone bills. The policy would have helped her group preserve affordable units, she said.

Fenway Forward twice made offers to buy a 55-unit apartment building in Boston’s Fenway neighborhood, with the support of tenants, but were outbid in 2019 and rejected in favor of a lower but faster-closing offer when it was put up for sale again in 2022. The group said Fenway Forward had planned to make the units affordable to households making between 30 to 60 percent of area median income, or between $40,000 to $80,000 for a two-person household.

The version of the policy that the House adopted would only have the measure take effect in cities and towns that opt in to the tenant purchase policy. Landlords would have seven days to notify tenants of a planned sale, and tenants would then have 15 days to form a tenant association and make an offer. If there is no offer from the tenant group or their partner, the landlord can find another buyer. Once an outside offer is made, tenants would have another 15 days — up to 30 in some circumstances — to match that offer, then 160 days to secure financing and close.

Landlords who own six units or fewer in a participating municipality are exempt.

The Senate version is narrower, creating a five-year pilot in five communities, with the Executive Office of Housing and Livable Communities writing the regulations.

The Massachusetts Law Reform Institute is part of the TOPA Coalition, a roughly 70-organization alliance of tenant groups, affordable-housing developers, and preservation nonprofits that has pushed the policy for more than a decade. Jochnick, the housing attorney, said the coalition would like to see the most comprehensive version make it through conference committee, but if it is a pilot, she would like to see at least 10 communities covered to include a wider array of cities.

MassLandlords, which represents owners and managers of rental property statewide, opposes the policy “in all forms.”

The group argues TOPA’s main beneficiaries are nonprofit community development corporations who would own the buildings, and that the added delay discourages sales altogether. It points to Washington, DC, as a cautionary tale. Lawmakers in the nation’s capital overhauled the statute last September, exempting multifamily buildings constructed in the past 15 years and some two- to four-unit buildings, along with adding a 45-day “cooling-off period” during which tenants can’t sell off their rights under TOPA to a third party.

For Edwards, the idea of starting out with a pilot program addresses a few major concerns. Because the state housing office would be in charge of regulations, there would be opportunities for feedback on the final program. It would also let a small number of willing Massachusetts municipalities be the testing grounds, and the five-year pilot could be renewed if the communities want to apply again and take more time to test the policy.

“If it doesn’t work, we’ll learn with limited damage,” she said. “If it does, then everyone wins.”