The hottest musical act in Philly? A group of middle-aged guys in khaki slacks and sensible loafers.

As a general rule, Sam Marshall is not used to drawing a crowd.

At 70 years old, with glasses and a plop of silver hair, the Haverford grandfather is a semiretired regulatory lawyer with a fondness for trains and the print version of the Wall Street Journal.

Yet, there he was last month, at a wedding in Big Sky, Mont., loitering near the dance floor when a young woman approached.

“You’re famous!” she said.

Before Marshall could respond, the woman was grabbing her friends and pulling them over.

“Hey!” she called out. “This is the guy from that group we follow!”

Such is life these days for the Tonics, a men’s a cappella group made up largely of Main Line fathers and grandfathers who might very well be the city’s hottest musical act.

Since videos of the group’s endearing kitchen rehearsal performances began circulating on social media recently — racking up millions of views — no one can seem to get enough of Philly’s a ca-fellas.

Online, the Tonics are putting up kittens-doing-cute-things numbers: 2.9 million Instagram views for a rousing rendition of “Lean on Me”; 4.1 million for a performance of “Werewolves of London.”

Supermodel Gigi Hadid is a fan. The singer Michael Bublé responded to their rendition of his song “Haven’t Met You Yet” with a string of heart emoji.

So meteoric has been their rise, meanwhile, that they’ve broken U.S. containment.

“Evidently,” says Mike Reese, a 46-year-old Wayne resident who sings second tenor for the group, “we’re very popular in Australia.”

The appeal is undeniable; in a digital world where every piece of content seems carefully curated, the Tonics have remained their unassuming selves.

Of the group’s 19 members, most are middle-aged and older — with full-time careers as doctors, lawyers, and educators. Their collective look — an assortment of khaki slacks, quarter-zip sweaters, and sensible loafers — might best be described as CPA chic. It is not uncommon, meanwhile, for one of the group’s members to pull a handkerchief from his pocket mid-performance and blow his nose.

During a Saturday afternoon performance at a restaurant in Wayne last month — where even the waitstaff couldn’t help clapping along — supporters clamored to make clear they’d been fans before the group’s mainstream success.

“I have all of their CDs,” said a woman named Esther, from her perch at the bar. “I put them on when I’m paying bills.”

Unlike many of their musical contemporaries, the Tonics did not set out for musical stardom.

Back in 1990, a couple of them — who’d previously been part of a cappella groups and missed the camaraderie — got to thinking: Why not get some guys together and do a little jamming?

They began meeting every Thursday night, rotating between members’ homes, where for two hours they could leave their stresses — family responsibilities, demanding careers — at the door and enjoy a little old-fashioned harmonizing with the boys.

“There’s something very spiritual that happens when you’re standing in a group and singing a song, making harmony together,” says Peter Evans, one of the group’s original eight members.

It wasn’t long before they were movin’ and snappin’ their way into the hearts of the masses — or at least the occasional graduation party, bat mitzvah, and assisted-living facility.

Through the years, they managed to achieve a modicum of local renown. They performed “The Star-Spangled Banner” at Phillies games. They recorded four studio albums. A catalog of their music — covers of hits spanning the decades — is available to stream on Spotify and Apple Music.

It wasn’t until earlier this year, however, that things started to get a ca-crazy.

During a rehearsal one evening last November, the group’s members stood around a well-appointed kitchen island, belting out a rendition of “Drift Away.” Someone captured a short video of the performance, and afterward, Youngmoo Kim — the Tonics’ 53-year-old music director — posted it to the group’s sparsely followed Instagram page, thinking little of it.

Maybe it was their wholesome vibe. Or the earnestness of their instrument-less performance.

Maybe, at a time when adult males — especially those over 50 — report feeling increasingly lonely, it was simply the novelty of a group of men gathering to do something that didn’t involve “golf and cigars and bourbon,” says Philip Pearson, a 57-year-old Tonic from Chestnut Hill.

“To be able to carve out a couple hours to do something special with a bunch of guys you like to hang out with is a pretty special thing these days,” adds Reese.

Whatever the reason, the Tonics struck a chord.

When Kim checked the group’s Instagram page a few days later, he was astounded.

“I’m used to seeing 300 views,” he says. “All of a sudden, it was 300,000.”

More videos followed, and soon, Instagram feeds across the world were filling with a dozen or so middle-aged guys from the Philadelphia area cutting loose around a kitchen island — their impassioned performances rivaled only by the stream of good-natured jokes that accompanied them.

“Room full of good credit,” read one comment on Instagram.

“A case of sparkling water hates to see these fellas coming,” read another.

“I just know they have a [favorite] length of grass.”

These days, everyone’s jockeying for a piece of the Tonics.

There were the folks from North Carolina who wanted to book the group for a summer wedding. And the woman from Chicago who begged the group to record a “Happy Birthday” video for her fiancé, a huge fan. (The Tonics — whose performance fee ranges from free dinner to $1,000 — agreed, but only after ensuring they weren’t being pranked.)

There have been collaboration requests and brand partnership offers, including one — much to the confusion of the group’s members — from a company selling skinny jeans.

“You’ve seen our shapes — our shapes are not all great,” says Jim Buck, 66, of Bryn Mawr, a cofounder of the Tonics. “But if they’re really looking to appeal to a bunch of 40-to-70-year-olds, then we’re your guys.”

The Tonics, of course, would not be the first band to succumb to the pressures and temptations that accompany overnight fame.

But while there has been the occasional sign of their newfound stature — “I do notice that maybe people comb their hair a little more before rehearsal,” says Marshall — the group’s members insist that they have remained cool and levelheaded about the whole thing.

Indeed, at a recent Thursday night rehearsal in a leafy Haverford neighborhood, there were no signs of swollen heads or raging egos. As they trickled in, the group’s members mingled and backslapped. They picked from an assortment of snacks — a family-size box of Triscuit crackers, some Di Bruno’s-sourced cheeses — and hammed it up in that uniquely suburban-dad kind of way.

“We got a photographer here tonight,” someone quipped. “Don’t pick your nose!”

“Bad-lookin’ guys in the back!” called another.

On this night, however, there was plenty to do. There was their suddenly packed calendar of gigs to prepare for — at the Woodmere Art Museum, the Wayne Music Festival, the Philadelphia Cricket Club.

And their forthcoming live performance on Australia’s The Morning Show.

And so, as the evening sunlight dwindled and the diet soft drinks flowed, the Tonics took their places around the kitchen island — van Gogh at his canvas — and got to work.