Thanks to a relentless news cycle — and a dedicated fan in the Oval Office — Fox News has defied the downward trends in the television business, notching its highest-rated year in 2017 even as audiences dwindled for many networks.

But the mass migration of viewers away from traditional cable and satellite packages is accelerating. And now Fox News is plotting a leap into the uncertain digital future that rivals like CNN have so far put off.

On Tuesday, Fox News is set to announce Fox Nation, a stand-alone subscription service available without a cable package. The streaming service, expected to start by the end of the year, would focus primarily on right-leaning commentary, with original shows and cameos by popular personalities like Sean Hannity.

It would not overlap with Fox News’s 24-hour cable broadcast — not even reruns — because of the channel’s contractual agreements with cable operators. Instead, the network is planning to develop hours of new daily programming with a mostly fresh slate of anchors and commentators.

“Fox Nation is designed to appeal to the Fox superfan,” John Finley, who oversees program development and production for Fox News, said in an interview. “These are the folks who watch Fox News every night for hours at a time, the dedicated audience that really wants more of what we have to offer.”

Mr. Finley said the network was still discussing the cost of a subscription.

The Fox News venture joins an increasingly crowded — and increasingly niche — marketplace for web-only streaming television.

ESPN is starting its subscription service, ESPN Plus, in the spring. About five million viewers signed up last year for HBO and Cinemax digital subscriptions. Last week, CBS said it counted five million subscriptions to its CBS and Showtime streaming services, and it plans to add two more stand-alone products, CBS Sports HQ and an offering branded for “Entertainment Tonight.”

Fox Nation, depending on its popularity, may prove more consequential to the country’s political life than the average streaming service.

Fox News already commands the attention of President Trump and many voters in his base. The digital product would bring viewers an additional dose of opinion programming beyond staples like “Hannity” and “Fox & Friends.” Live events, like question-and-answer forums, would encourage more direct interaction with anchors and commentators.

Fox News viewers “value our product so much, they go to hotels and if they can’t have Fox, they send us emails. They go on cruises, and if they can’t have Fox, they send us emails,” Mr. Finley said. “This is a way for us to meet that demand.”

Whether the venture would be a moneymaker is up in the air.

Fox News reaps more than $1 billion in annual profit, providing ample funds to hire a new team for Fox Nation, which is not expected to initially carry advertising. Mr. Finley declined to estimate his start-up costs, and streaming services in conservative media have had a mixed record of success.

The Blaze, a web-only service founded by the host Glenn Beck in 2011 after he left Fox News, struggled to attract interest and eventually morphed into a more traditional network distributed by cable and satellite providers. Bill O’Reilly, who was fired by Fox News in April, started a subscription service on his website that has earned little attention.

Mr. Finley said Fox Nation was not comparable to a personality-driven product. “This is not starting from scratch here,” he said. “Glenn Beck had a ton of viewers when he was here on Fox. When he left, it didn’t seem to me that they followed him. People are loyal to the Fox brand.”

The median Fox News viewer is 65 years old, according to Nielsen, but the network points to its website traffic and heavy presence on Facebook and other social media platforms as evidence that a web-only service can appeal to its audience.

Among Fox News’s main rivals, MSNBC has no stand-alone product. CNN has a streaming service, CNNgo, which offers some free original programming, but it otherwise requires an existing cable or satellite subscription. Jeff Zucker, CNN’s president, said in December that he was considering a digital product for the channel’s “Great Big Story” brand, which is aimed at younger viewers.

Fox News, though, is facing some new competition on its conservative flank. The potential expansion of the Sinclair Broadcast Group may bring more conservative programming to local television stations. Peter Thiel, the technology investor and Trump supporter, is said to be interested in creating a right-leaning media organization based in Los Angeles.

Asked if Fox Nation was a response to pressures from cord-cutting and other industry trends, Mr. Finley said Fox News loyalists “are not cutting the cord anytime soon.”

“I don’t think this is about competing with our rivals. It’s about serving our audience,” he added. “We know who our audience is. We know what they want.”