CommonWealth owns over 300 office properties in the U.S., including 109 Brookline Ave. in Boston. CoStar Group

The widespread belief that activist campaigns and hostile takeovers can't win in the world of real-estate investment trusts may soon be upended.

CommonWealth REIT, which owns more than 300 office buildings in the U.S., is under siege from a group of dissident shareholders, led by billionaire real-estate magnate Stephen Ross's Related Cos. and hedge-fund firm Corvex Management LP. The investors are seeking to replace management, arguing it is rife with conflicts of interest that have kept CommonWealth trading at below the value of its office-property portfolio.

"For a long time, nobody thought a REIT could be successfully attacked, so nobody bothered," said Darren Novak, a banker at Houlihan Lokey who advises companies and activist investors. "If CommonWealth falls, that perception will change."

Activist battles and hostile takeovers of REITs are rare, partly because of tax rules and corporate charters that prevent any one investor from owning more than about 10% of a company's shares.

REITs' hefty dividends also have shielded them from a common argument made by activists—that companies aren't returning enough cash to shareholders. Moreover, most REIT stocks typically trade at or near the value of their properties, which makes it hard for activists to argue they can uncork hidden value.

Of hundreds of board battles waged at public companies since 2006, only about a dozen have targeted REITs, according to data compiled for The Wall Street Journal by FactSet. No dissident has swept a vote or won a majority of seats. A few have negotiated settlements for one or two board seats. Most lost or gave up.

CommonWealth has been criticized because its management has earned fees based on the size of CommonWealth's assets, rather than on the company's performance. The dissidents say that encourages the company to acquire more office buildings, no matter the price, and to sell aging properties rather than renovate them. Analysts at Green Street Advisors called CommonWealth "uninvestable" last year because of its management structure.

The CommonWealth dissidents' argument has gotten enough traction that they might succeed in spite of the share-ownership rules by cobbling together a critical mass of sympathetic shareholders. Corvex, run by Keith Meister, a disciple of activist investor Carl Icahn, and Related persuaded owners of 70% of shares to vote to oust CommonWealth's board last year.

An arbitration panel voided that vote on procedural grounds late last year. But the panel's ruling paved the way for the new vote that started on Feb. 20 and concludes in about three weeks. The outcome is too close to call.

CommonWealth is managed by Reit Management & Research, a Newton, Mass.-based external advisory firm run by father and son Barry and Adam Portnoy. Because CommonWealth technically has no employees of its own, RMR provides management services for a fee, which for years has been based on the value of the REIT's assets.

Adam Portnoy, chief executive of RMR, said in an emailed statement Monday that Related and Corvex "are pursuing a self-interested agenda" and have a plan for CommonWealth that is "short term oriented, reckless and inconsistent with the investment goals of long term REIT investors." He also accused the dissident group of a "campaign to take control of CommonWealth without paying shareholders anything for that control."

In the latest twist in the CommonWealth fight, the dissidents brought on Sam Zell, the billionaire Chicago real-estate mogul, to join their proposed slate of directors as chairman.

Adam Portnoy has criticized an arrangement that gives Mr. Zell and one of his lieutenants, David Helfand, the option to buy up to four million shares of the REIT's stock currently held by Related and Corvex at a discounted price, saying it makes him "question [Mr. Zell's] motivations" for joining the board.

The Portnoys have been working the phones and traveling to New York to court shareholders, according to investors who have heard their pitch.

Over the past few months, CommonWealth also has made a series of governance changes as a response, in part, to the activist battle mounting against them, including restructuring the board so that directors aren't all chosen at once, adding new seats for independent directors, promising to drop a poison-pill defense and changing the way management fees are calculated by tying them to the company's stock performance.

Related and Corvex have said these changes are "window dressing" and can easily be abandoned if CommonWealth wins the shareholder vote.

"Clearly these changes were reactionary to the pressure they were under," said Ian Ferry, a portfolio manager at Delaware Investments, a mutual-fund manager that owns 8.13% of CommonWealth's stock and is siding with the dissidents. "All that anyone's asked for is a vote. Everything they've done has been to obstruct that."

The outcome of the vote hinges largely on a few shareholders. The Vanguard Group Inc., owns more than 12% of CommonWealth's stock, while affiliates of BlackRock Inc., own about 7%. Vanguard sided with the dissidents in last year's vote; BlackRock abstained. Both firms declined to comment.

In 2012, the Portnoys' company earned $65.3 million in fees from CommonWealth, which has a market capitalization of $3.15 billion, while CommonWealth's stock trailed the Nareit Office Index by 9.2 percentage points.

Through the first three quarters of last year, Reit Management has received $48.6 million, compared with $48.9 million over the same period in 2012, the company said. Over the first three quarters of 2013, however, starting around when the dissident group announced it was accumulating shares, the company's stock has beaten the Nareit index by 38.7 percentage points.

The dissidents and other critics also have attacked CommonWealth for reappointing a director one day after receiving just 14% of votes at a May shareholder meeting, and for requiring shareholders to arbitrate disputes confidentially rather than sue in open court. "The corporate governance track record of Portnoy-managed companies isn't pretty," says Ann Yerger, executive director of the Council of Institutional Investors, a Washington-based nonprofit that focuses on shareholder rights.

CommonWealth said that the director was reappointed because his ouster appeared to be related to Related's and Corvex's proxy battle rather than any personal failings.

If the Related/Corvex group wins, analysts say the most vulnerable targets will be other REITs that, like CommonWealth, are managed by external companies. There are only 17 of them—excluding those that invest in mortgage bonds—of 162 publicly listed REITs. Five are controlled by the Portnoys.

Write to Robbie Whelan at and Liz Hoffman at