In the first tweet, Mr. Lopez said, "I am against #VoxUnion." He followed that one by arguing that some writers wanted a union "as protection for laziness." He added that Vox was "a generous company" and "some people want to take advantage of that."

After employees at the online media company Vox Media announced plans to form a union last month, German Lopez, a senior reporter at the company's general news website Vox.com, posted a thread on Twitter that inspired a heated debate more than 1,000 comments in length.

Then a couple of weeks ago, Mr. Lopez changed his mind, swayed by the idea that the company's generosity isn't set in stone and that new management could roll back benefits.

The organizing effort at Vox is part of a growing union movement at more than a dozen digital publishers. When they entered the media fray, these and other online players sought to shake up the status quo, but while their traffic soared, their profits didn't. Now they confront the same job cutbacks and financial uncertainty as their more established brethren.

Employees at now-defunct Gawker Media joined the labor union the Writers Guild of America East in the summer of 2015. Members of the editorial staffs at Vice Media, ThinkProgress and HuffPost followed suit, organizing unions that their companies recognized and that subsequently ratified contracts.

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Writers and editors at other digital publications — including The Intercept, Salon, Slate, Thrillist and MTV News — have since affiliated themselves with the Writers Guild of America East, citing a need for better wages and benefits. Their unions have been recognized by the people who run those companies and are negotiating contracts with management.

Daniel Marans, a reporter at HuffPost, said the treatment of employees at digital media companies should not remain stuck in a time when websites were small and scrappy, staffed by younger workers who were happy to see their names in pixels.

"That comes to things like transparency on pay, having a decent pay scale that allows a ladder of sustainability where you can support yourself on such an income, and having due process and a guarantee of severance in the case of layoffs," Mr. Marans said.

Kim Kelly, an editor at Noisey, Vice Media's music and culture section, said fair wages had played a central role in Vice Media employees' decision to go union.

"People were fed up and broke and anxious about the future, and the union gave them a way to take control and force things to change," Ms. Kelly said.

It's a labor model that has been in place for years among the big media operators. , Reuters and The Wall Street Journal all have unions.

That digital players are moving in that direction reflects the broader business troubles in the media industry as a whole. Digital writers and editors live with the uncomfortable possibility that their bosses could enact a sudden change in strategy, resulting in layoffs.

In recent months, 50 employees at Mashable were let go after the digital publisher Ziff Davis bought the website for $50 million, BuzzFeed fired 100 business and British editorial employees after missing its 2017 revenue targets, and Refinery29 laid off 34 staff members.

Lowell Peterson, the executive director of Writers Guild of America East, denounced the tendency of executives at online media companies to make important decisions concerning company strategy without the input of employees.

"They just open their email inbox and go, 'Oh, dear, what next?'" he said.