The mouthwash industry has a message for America: You're really not using enough, and it shows.

Sure, the oral-care megabrands – Procter & Gamble Co.'s Crest and Colgate-Palmolive Co.'s flagship – have extended into mouthwash in recent years and put forward the "regimen" idea that mouthwash is an integral part of oral care. And all the players in the category have piled on multi-benefit products that whiten or strengthen teeth even as they freshen breath and kill germs.

But that's resulted mainly in share shifts, not growing the business. While toothpaste and toothbrush usage is nearly universal, household penetration of mouthwash has been "stuck in the 40s" percentagewise for years, said Larry Page, senior marketing director for Listerine at Johnson & Johnson.

So Listerine recently launched a "Heroic Habits" campaign on TV, print and digital. Led by Health@JWT, the push includes its own YouTube channel aimed at improving overall oral health and focused specifically on Listerine mouthwash as part of the solution.

People believe (J&J would say wrongly) "that brushing alone is enough," Mr. Page said. But data show about half of adults have some kind of gum disease (presumably from the more than half not using mouthwash). That's proof to him that folks need mouthwash in their lives, particularly of the germ-fighting Listerine variety.

Listerine ads focused on mouthwash as part of an overall healthy campaign broke in July, followed by the YouTube channel in September, which got more than a million views in its first three weeks. Videos show people adding mouthwash to their routines at home, and others going through their oral-care routines at a shopping mall as a dentist behind a two-way mirror reveals himself.

"As the category leader we feel the responsibility to improve oral health," Mr. Page said. Other competitors who've launched into the category seemingly should have grown the market, he said, but they "haven't forced people to think about things differently, so that's why we're taking a different approach."

So far, J&J's effort doesn't seem to have changed the trajectory of the category either – mouthwash sales were up 1.6% in the latest four weeks ended Sept. 27, according to Nielsen data from Deutsche Bank, vs. a 2.6% gain for the full 52 weeks. J&J's approach has, however, added 1.3 and 0.7 points to its market share in mouthwash for the latest four- and 12-week periods, respectively, starting to reverse a 1-point loss for the full year driven by Colgate's gains.

Meanwhile, the other top brand rooted firmly in mouthwash as opposed to broader oral-care – P&G's Scope – is taking a back-to-basics approach to regain its edge in new ads from PublicisKaplanThaler, New York.

Scope has had a rough couple of decades, declining fairly consistently over the years amid gains from Listerine, Colgate's other specialty rinses such as Chattem's Act, and even its own sibling Crest. That's led the brand to try many ways to reinvent itself – most recently taking on a fun brand personality that included ads featuring hot kisses in mosh pits and a faux bacon-flavored variant for April Fool's Day in 2013.

None of that arrested the decline. So the latest ads take a more direct approach, not unlike the brand's ads from the 1960s. They show breath so bad it makes cats cry and mustaches fall off men's faces, until alleviated by Scope. The brand also brought back a Scope Classic label and bottle nationally in July and made the retail margin for Scope more attractive after falling behind rivals, according to people familiar with the matter.

Scope Classic, focus of the new ads, already has captured more than a 2% share of the $1.4 billion category, according to IRI data for the 52 weeks ended Sept. 7. But that's not enough to reverse P&G's overall 4% sales decline and 1.7-point share loss, as Crest sales have been hurt by launch of Colgate mouthwash.

This could be Scope's last chance to keep its place as a P&G brand. Chairman-CEO A.G. Lafley announced in August that the company plans to cull more than 100 brands that are underperforming. And Scope, overshadowed by big-brother Crest, is a prime candidate for divestiture, according to some analysts. P&G declined to comment.

If Scope goes, it might find some willing buyers. Usage challenges or no, mouthwash has been one of the 10 healthiest household and personal-care categories in terms of showing both positive volume and pricing in the past year, according to a recent Deutsche Bank analysis. Should marketers truly convince even a small sliver of the majority of Americans who aren't using mouthwash to do so, it could become more attractive still.