WARSAW — By the standards of Western advertising, Coca-Cola’s billboard campaign in Hungary was pretty tame.

Three couples are shown enjoying a soda, smiling and seemingly in love. One picture shows a man, a woman and a Coke; another two women and a Coke; and a third shows two men and a Coke.

“Love is Love,” is the campaign slogan. But in the current climate in Eastern and Central Europe, where “L.G.B.T. ideology” has taken the place of migrants as public enemy number one for many nationalist leaders, love is not love.

It is a threat.

Soon after the Coke ads appeared, a pro-government internet news site ran a banner headline: “The Homosexual Lobby Has Now Besieged Budapest — They Won’t Give You A Chance to Avoid It.”

Istvan Boldog, a lawmaker representing Prime Minister Viktor Orban’s far-right Fidesz party, used Facebook to call on the public to boycott Coca-Cola products until the company “removed its provocative posters from Hungary.”

The battle over the billboards is just a small skirmish in what is emerging as a broader campaign across the region against gay rights. Right-wing politicians complain that their traditional cultures are undermined by a decadent and dangerous import from the irreligious West.

In 2013, Russia made it illegal to expose minors to discussion of “nontraditional” sexual relationships.

More recently, Poland’s leaders have focused attention on what they call “L.G.B.T. ideology,” painting it as an insidious threat to the nation. Other parties in the region are watching closely to see how effective it proves.

In the run-up to national elections in October, Poland’s governing Law and Justice Party, along with Catholic Church leaders, have stepped up their attacks. More than two dozen provincial governments have declared their localities “L.G.B.T.-free,” and the party leader, Jaroslaw Kaczynski, has said Poland will not live under “the rainbow flag.”

The vitriol in Hungary, Poland and other countries bears striking similarities to the region’s vehement reaction against the wave of migration into Europe that peaked in 2015, as people fled war and deprivation in the Middle East and Africa.

Mr. Orban was at the vanguard of painting a frightening image of a continent under siege by terrorists and parasites. His relentless campaign — often using billboards targeting political enemies — proved politically effective, and others across the continent would take a page from his playbook.

Then the flow of migrants slowed dramatically, and few settled in Eastern Europe. The issue lost much of its potency, but the campaign against gay rights offered a new group of people to paint as a threat.

“Kaczynski’s and Orban’s populism provides a dangerous cocktail of anti-pluralist, strongly xenophobic features which are then followed by legal, systemic changes in both countries,” said Edit Zgut, a visiting lecturer at the Center for Europe at University of Warsaw. “Fear mongering against inner enemies usually pays off politically as it channels voters’ frustration toward the most targetable social groups.”

After marchers at a gay pride parade in the conservative town of Bialystok, Poland were attacked in July, critics of the government said that the propaganda was fueling violence.

That mirrors a debate taking place in the United States, where President Trump’s opponents say that his fevered warnings about an invasion of immigrants were emboldening extremists and fueling violence like the recent massacre in El Paso.

On Monday, YouTube blocked an account belonging to a far-right Polish anti-abortion group, The Life and Family Foundation, for promoting “content glorifying or inciting violence against another person or group of people.”

In the immediate wake of the attacks in Bialystok, Polish politicians sought to distance themselves from the more hateful rhetoric. But the campaign against “L.G.B.T. ideology” has not slowed.

Every criticism of the campaign, in fact, is used as evidence that those promoting gay rights are part of some sinister cabal looking to undermine traditional values and national sovereignty.

The Archbishop of Krakow, Marek Jedraszewski, compared the L.G.B.T. movement to Communism during a sermon last week.

“The red plague is no longer on our land,” he said. “But it does not mean that there is not a new one that wants to rule our souls, hearts and minds. It is not Marxist or Bolshevik, but it has been born of the same neo-Marxist spirit. It’s not red, but rainbow.”

The conservative radio station Radio Maryja posted the homily on YouTube, prompting a formal warning from YouTube for “spreading hate.”

While there have been stepped-up campaigns against L.G.B.T. rights in other countries, it remains to be seen if they prove politically potent.

Same-sex marriages are legal in Scandinavia and most of western Europe, and civil unions are allowed in many other countries on the continent. But Romania, like Poland and Russia, is among a handful that do not allow either.

In October 2018 Romanian politicians organized a referendum on whether to narrow the constitutional definition of a family to a man and a woman, rather than the gender-neutral term “spouses,” which conservative groups feared could lead to legal recognition for same-sex relationships in the future.

Despite strong support from the country’s governing Social Democratic Party and the Orthodox Church, the primary response was apathy, and the referendum failed. More voters approved than disapproved, but turnout was just 20.4 percent, far below the 30 percent required for it to take effect.

In Slovakia, another nation that does not recognize gay couples, a popular magazine widely known for spreading disinformation, Zem a Vek, offered stark warning. On its cover this week, it featured a photo of guillotine, painted in rainbow colors, with the headline: “L.G.B.T. Terror.”

Prominent far-right and ultranationalist politicians regularly attack L.G.B.T. people in their speeches, including Andrej Danko, the speaker of Slovak Parliament and the leader of the Slovak National Party.

But the new Slovak President, Zuzana Caputova, won a resounding victory in March after campaigning on a platform of tolerance and support for gay rights. And the pride march in the capital, Bratislava, in July drew the largest crowd in its history, with 10,000 people filling the streets.

Private companies, many with global reputations and brands to protect, have also pushed back against overt bigotry.

Coca-Cola refused to take down its billboards in Hungary, defending its campaign as being in line with its corporate values. “We believe both hetero- and homosexuals have the right to love the person they want, the way they want,” the company said.

On Wednesday, it announced that the posters would be replaced with images of Coca-Cola bottles with rainbow-colored labels, which company officials said was always the plan.

The American Chamber of Commerce branch in Hungary said it stood behind the right of companies like Coca-Cola to express their views.

“We believe inclusion, tolerance and openness are essential to a modern, progressive society, and promoting equality is pivotal to economic growth and competitiveness,” the group said.

In Poland, Empik, the largest book and media retail store, withdrew an issue of a right-wing newspaper, Gazeta Polska, that included “L.G.B.T.-free zone” stickers.