Sure, craft beer is on an incredible run. Hard cider is cool, and fruity ales are having their day.

Now here comes hard soda.

Fueled by buzz generated by liquored-up root beers—most notably Small Town Brewery's Not Your Father's Root Beer—big brewers are jumping into the category.

The latest is MillerCoors. The Chicago-based joint venture best known for Miller Lite and Coors Light plans to start selling Henry's Hard Ginger Ale and Henry's Hard Orange sodas in six packs of 12-ounce bottles (suggested retail price $8.99) and in 16-ounce individual cans to retailers across the country in January. Backed with a national ad campaign that will feature spots on television and digital marketing, MillerCoors is aiming its products at Gen Xers who grew up drinking soda and now have more disposable income to spend on boozy alternatives.

The brewer over the past decade has hemorrhaged share of the U.S. beer market to an ever-growing army of craft competitors whose combined share of the beer market crossed double digits for the first time in 2014. In response, MillerCoors has focused on expanding its craft offerings under its Blue Moon and Leinenkugel's banners and branched into fruit-forward beers with Redd's Apple Ale and cider with its growing Smith & Forge line.

Now it sees a huge opportunity in hard soda, a category that, save root beer, largely remains untapped. "This is a true white-space opportunity in the marketplace," says Bryan Ferschinger, MillerCoors' director of innovation. "We're seeing very strong trends in craft sodas and other flavorful offerings with alcohol, and we see huge consumer appeal that will allow this to be a strong national play out of the gates."

Both sodas are sweetened with pure cane sugar and clock in at a meager 4.2% alcohol by volume, in line with light domestic beers. While initially available in only two flavors, Ferschinger says more are in the works. "That's why we're calling it the launch of our hard soda platform," he says. "Everything we see leads us to believe this is not a flash in the pan."

If history is a guide, however, hard soda's staying power is up against fairly long odds. Remember Zima? How about Smirnoff Ice? Both brands shot out of the gate with a vengeance before sales eventually cooled, and in Zima's case disappeared altogether in the U.S.

Even MillerCoors competitor Anheuser-Busch InBev's Bud Light Lime-a-Rita, a margarita-flavored beer that launched in 2012 to wild success, recently collapsed. Sales of the company's beverage coolers, which include the Lime-a-Rita family of products, are down 9.7% in the 12-month period ended July 12, according to Chicago-based market research firm IRI.

"The big open question is whether they can break the cycle," says Bart Watson, chief economist of the Brewers Association, a Boulder, Colo.-based trade group. "Every time a new brand or a new flavor is introduced, sales rocket up, but they tend to fall down just as hard."

Mike's Hard Lemonade, which launched in 1999 with its namesake brand, rose to national prominence by positioning itself as an alternative for beer drinkers. The Chicago-based brand, now in its 16th year, has more or less bucked the trend with continuous innovation. Now with more than 30 products, including obscure flavors like hard blood orange, sales continue to rise.

When sales of its flagship line started to slow—revenue is down 4.3% to $239.3 million over the 12 months ended July 12, according to IRI—Mike's Hard found another way to differentiate: an amped-up offshoot called Mike's Harder. This 8% alcohol-by-volume version has outperformed the rest of the category by a wide margin, with sales rising 18.3% over the same period to $194.4 million, IRI says.

Spiked root beer is riding high, too, for now at least. Wauconda, Ill.-based Small Town brewed its first batch of hard root beer in 2013 and sold it mostly to Chicago-area bars. When Not Your Father's Root Beer hit Binny's stores around the holidays last year, at $9.99 a six pack, "we were holding on for dear life hoping to keep it in stock," says Pat Brophy, beer buyer for the Chicago chain. "To my pleasure, it's continued. It's just tremendous growth."

Although sweet enough to lump in with the hard soda or flavored malt beverage category, Not Your Father's is technically an ale, and many stores stock it among craft beer. It is the top-selling beer at many Binny's outlets, Mr. Brophy says. Its rise has nudged sales of other hard root beers, such as Sprecher's of Glendale, Wis., and Chicago's Berghoff Rowdy Root Beer, and spurred the launch of newbies, like Coney Island Hard Root Beer from Boston Beer, maker of Samuel Adams beer.

In another sign of the thirst for hard root beer, Small Town recently sold the Not Your Father's brand to a group of investors that include Pabst Brewing CEO Eugene Kaspher.

Mr. Brophy says hard root beers and hard sodas have a chance to outlast boozed-up pop predecessors because they play on nostalgia and appeal to beer drinkers and non-beer drinkers alike. Another key, he says, is marketing to both men and women, a tactic employed successfully by Not Your Father's.

But MillerCoors, which once made Zima, thought it had something that would last, too.

Peter Frost is a reporter for Crain's Chicago Business