SEOUL—"We beg of you, meet us in person," a Samsung employee pleaded over the apartment intercom to Kim Dong-chil, a retiree with about 0.004% worth of Samsung C&T Corp., the construction and trading arm of South Korea's biggest conglomerate.

The Samsung employee, who waved his business card in front of the intercom camera to prove his credentials, had appeared at Mr. Kim's doorstep for a simple reason: to persuade Mr. Kim to support the $8 billion merger of Samsung C&T and Samsung's de facto holding company, Cheil Industries Inc., at a shareholder vote this Friday.

The all-stock merger, announced in May, is a crucial step in a once-in-a-generation succession plan designed to secure control of Samsung Electronics Co., the crown jewel of the Samsung conglomerate, in the hands of heir apparent Lee Jae-yong, son of chairman Lee Kun-hee. The elder Mr. Lee remains incapacitated after a heart attack last year.

Samsung C&T owns a 4.1% stake in Samsung Electronics.

Mr. Kim, whose 7,000 shares in Samsung C&T are worth about $400,000, told the employee that he didn't want to be bothered. But he said the Samsung employee kept coming back, buzzing his apartment again and again, saying he wanted to explain the rationale for the merger.

Home visits, text messages, front-page newspaper advertisements and even hand-delivered watermelons are Samsung's weapon of choice in its bitter war against U.S. hedge fund Elliott Associates LP. In recent months, the New York-based firm has spent about $1 billion accumulating a 7.1% stake in Samsung C&T and small positions in two other listed Samsung affiliates with stakes in Samsung C&T, according to a person familiar with the matter.

Elliott's attack indirectly hits at a little-known vulnerability in the family's grip on the Samsung conglomerate of some 70 companies: The younger Mr. Lee's direct stake in the world's biggest smartphone maker is just 0.57%.

Lee Jae-yong is bound to inherit his family's stakes in Samsung Electronics—Lee Kun-hee alone owns a 3.4% direct stake—and other affiliates, but the inheritance tax bill of more than $5 billion he is expected to face could force him to liquidate some of his positions.

The Lee family hasn't spoken publicly about the merger and declined to comment through a spokesman for this article.

Using the Lee family-controlled Cheil Industries to swallow Samsung C&T is the cheapest and fastest way for the younger Mr. Lee to take control of Samsung C&T's 4.1% stake in Samsung Electronics, analysts say.

The effort by Samsung to win over Mr. Kim and other minority shareholders underscores the great lengths to which Samsung is going to ensure that it wins the necessary two-third majority at Friday's too-close-to-call shareholder vote.

Despite Samsung's quick rise in reaching the ranks of top global brands, the conglomerate has yet to convince many shareholders that its corporate governance, transparency and shareholder policies are on par with its global peers and that the younger Mr. Lee is ready to succeed to the throne. The outcome of Friday's vote will show just how far Samsung and the younger Mr. Lee have come in earning the respect and admiration of the global investment community.

If the merger fails, Samsung could find itself mired in a broader struggle for control of its affiliates. Elliott has suggested that it will fight to replace directors on Samsung C&T's board if the deal falls through. In addition, Elliott's 1% stakes in the two affiliates, Samsung Fire & Marine Insurance Co. and Samsung SDI Co., give the U.S. hedge fund room to sue on behalf of those companies, according to people familiar with the matter.

Elliott, which believes Cheil Industries' offer undervalues Samsung C&T, has already filed a pair of court motions seeking to block the merger, though South Korea's court has sided with Samsung on both cases—decisions that Elliott has appealed.

Samsung C&T shares are up 21% since the $8 billion merger was announced on May 26, giving it a market capitalization of 10.47 trillion won ($9.1 billion).

A failed merger would also leave the younger Mr. Lee with no clear path to increasing his influence over Samsung Electronics at a time of profound challenge for the smartphone maker, whose leadership has been threatened by the rise of Chinese handset makers and the popularity of Apple Inc.'s devices.

Even if Samsung pushes through the merger, Elliott's challenge will ensure a more difficult, and costly, transfer of power for the Lee family as other U.S. hedge funds flock for investment opportunities.

"Hedge funds are going to be watching every move," said Kim Woochan, a professor of finance at Korea University who says he has been peppered with calls from foreign investors interested in Samsung's restructuring.

Left with less favorable options, some analysts say the Lee family's best option if the deal fails is simply to try to strike the deal again at a richer premium. South Korean law doesn't allow a company to sweeten a merger proposal once it has been made, which means an entirely new merger proposal would have to be submitted—a process that Samsung executives have publicly sworn off.

The Lee family may also seek to combine Samsung Electronics with another affiliate in which the younger Mr. Lee holds a substantial stake, analysts say.

Ahead of the Friday shareholder vote, Elliott has called the merger "grossly unfair" to Samsung C&T shareholders and has so far garnered at least 9% of the vote on its side. International investors, including Aberdeen Asset Management, the Canada Pension Plan Investment Board, the California State Teachers' Retirement System and the Florida State Board of Administration, have sided with Elliott. Domestic shareholder Ilsung Pharmaceuticals Co., which owns 2.1% of Samsung C&T, has also signaled that it will oppose the merger.

Proxy-advisory firms Institutional Shareholder Services Inc. and Glass Lewis & Co., as well as two Korean counterparts, have also advised shareholders to oppose the deal, citing a lack of transparency and poor financial terms for minority shareholders. Meanwhile, Samsung has argued that the deal would boost long-term shareholder value, while warning Samsung C&T shareholders to "be wary of Elliott's questionable motives."

The Samsung C&T stakes owned by Samsung affiliates, the elder Mr. Lee and construction-materials company KCC Corp.—which bought a stake in Samsung C&T following the rift with Elliott— put Samsung's stake slightly ahead of Elliott, at 20%.

Samsung also appears to have won the support of its single largest shareholder, South Korea's National Pension Service. The world's third largest pension fund owns an 11.2% voting stake in Samsung C&T. A spokeswoman for NPS has declined to comment on how it will vote.

At this point, neither party appears to have enough votes to sway the outcome of Friday's vote. That turns the focus to foreign shareholders—excluding Elliott—who own 26% of Samsung C&T shares and the Korean mom-and-pop investors, who collectively hold 24% of the company. To win their support, Samsung has dispatched rank-and-file employees in the past two weeks to woo small shareholders like Mr. Kim, who says he plans to oppose the merger.

Another shareholder said in an interview that a Samsung C&T employee left his business card—and a watermelon—with his apartment's security guard, along with a handwritten plea to approve the deal.

"In accordance with all legal requirements, Samsung C&T has been reaching out to our shareholders to support the merger which we strongly believe to be in the best interest of the company and our shareholders," a Samsung C&T spokeswoman said.

Samsung has sent its top executives overseas in a bid to sway foreign institutional investors. Mr. Lee, the 47-year-old Harvard-educated heir apparent, has even invited an outspoken opponent of the merger for tea at Samsung's headquarters, according to people familiar with the matter.

On Monday, Samsung bought front-page advertisements in all of South Korea's major newspapers to rally support to its side.

"We desperately plead with Samsung C&T shareholders," the ad read, calling it "regrettable" that Elliott was seeking to block the proposed merger. "Must the future of Samsung C&T and Cheil Industries be hindered like this?"

Samsung C&T has also hired bankers at Goldman Sachs Group Inc. and Credit Suisse AG to advise on the deal—a rare move for a conglomerate that usually handles its deal-making in-house. Separately, Samsung pledged to bolster corporate governance by boosting dividends and creating a shareholder rights committee at the new company—provided the merger is first approved by shareholders.

Elliott is fighting back. In recent weeks, it has found an unusual level of sympathy among local shareholders of Samsung C&T, thousands of whom have banded online to discuss the merger.

While the shareholders have sworn independence from both Elliott and Samsung, many are nonetheless opposing the deal, posting photos of their proxy voter forms before mailing them to Elliott.

"I was disappointed at Samsung's attitude, which deceives small shareholders," one shareholder who delegated her vote to Elliott wrote in an email. She said she was harassed by Samsung C&T employees.

In times past, an attack on the most powerful family in South Korea's clubby corporate sector—in a country where family-run conglomerates, or chaebol, dominate the economy—wouldn't have gotten very far. The Lee family, led by its ailing chairman, is seen as untouchable in a country dubbed the Republic of Samsung because of its interests in technology, construction, insurance, securities trading, fashion and lifestyle.

Samsung Electronics alone accounts for about one-fifth of South Korea's stock market value, and seven Samsung-affiliated companies populate a new Dow Jones Industrial Average-style stock index of 30 constituents created by Korean regulators.

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While victory is far from ensured for interlopers like Elliott, the vote's outcome could redefine how shareholder activism is viewed in a country long seen as one of the most inhospitable destinations for foreign capital in the developed world.

Family succession, once regarded as the norm in South Korea, is coming under criticism by ordinary Koreans. There are signs that many shareholders are starting to react more positively as outside investors focus attention on corporate-governance questions that have rarely been raised.

Hugh Young, director of Aberdeen Asset Management PLC, which owns less than a 0.1% stake in Samsung C&T, said he had no choice but to oppose the deal.

"If one does not protest, it could be seen as endorsing poor practices," he said. While South Korea isn't the only country with controlling shareholders who are looking out for their own self-interest, he said: "Given the predominance of chaebol it is arguably a larger issue in Korea."

Min Sun Lee contributed to this article.

Write to Jonathan Cheng at jonathan.cheng@wsj.com and Min-Jeong Lee at min-jeong.lee@wsj.com

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