Getting their Siobhan on

Magnus, the 20-year-old Marstons Mills native who’s making a name for herself nationally as an “Idol’’ finalist, would go on to place among the top nine last week, after delivering what many felt was her least impressive performance to date. “Idol’’ judges were notably harsh on Magnus Tuesday, calling her effort weak and uneven. Stephanie Gianno, Magnus’s best friend, who was sitting at the VIP table, stated what seemed obvious, if momentarily disconcerting, following Magnus’s subpar performance. “It wasn’t Siobhan’s best night,’’ said Gianno. “But she’ll be back. Besides, she can’t steal the show every week.’’

Maybe not. But all over Cape Cod these days, in pizza joints, post offices, and pet stores, Siobhan-o-mania has erupted in full force. Down here, she is the show.

Bumper stickers on cars from Chatham to Mashpee urge fans to cast a supportive vote for Magnus. The marquee at the Cape Cod Melody Tent boasts a similar message, as does a sign outside Cape Maid Farms, a Hyannis pet supply store. Manager Kelley King said the store often posts topical messages, usually with a patriotic or sports theme. Saluting Magnus was only natural, said King, not just because Magnus’s father, Alan, once worked at the store but because she’s “part of this community, someone we’re all proud of.’’

Magnus is very much known as a local girl around here, having graduated from Barnstable High School, where she starred in numerous theatrical and musical productions, and having performed with Cape-based rock bands like Lunar Valve. Many who saw her sing “Tomorrow’’ from the musical “Annie’’ as a grade-schooler predicted future stardom for her, and now that scenario grows more plausible with each passing week.

A fresh generation of grade-schoolers certainly seems to think so. At the West Village Elementary School in Marstons Mills, the entranceway has been lined with pro-Siobhan posters for weeks. Students, teachers, and staffers all wore blue tops to school Tuesday — blue being the official “Idol’’ color — and posed for a group picture they’re sending Magnus as a good-luck totem. According to school administrator Ellen MacDonald, on the mornings after “Idol’’ airs, “everyone here is exhausted from staying up watching.’’ That includes many of the youngest children, she added, two of whom are Magnus’s sisters.

West Village students also made a collage of “Go Siobhan!’’ signs that hangs in the Marstons Mills post office, where postmaster Mary Beth O’Brien has been collecting townspeoples’ signatures on a scroll, a future Siobhan keepsake. “This is a small town, and it’s a real positive for something like this to be happening,’’ said O’Brien, noting that she’s already voted “multiple times’’ herself for the hometown favorite.

The local-girl-makes-good angle is always compelling, and no doubt other “Idol’’ contestants have their own rabid fan bases. In a telephone interview, Magnus said the support she’s gotten from Cape-based fans has been particularly gratifying, though, as the pressure intensifies. “The most inspiring thing is getting letters from favorite old teachers, saying how proud they are,’’ Magnus said. “It comes full circle, because I wouldn’t be here now if it weren’t for them.’’

Considered a long-shot going into Top 20 competition on “Idol,’’ Magnus has consistently impressed the judges — and viewers — with her soulful, sometimes unconventional rendition of songs like Chris Isaak’s “Wicked Game’’ and Stevie Wonder’s “Superstition.’’ Known for her ability to hit big notes and sustain them, Magnus has faced criticism lately for becoming too gimmicky, a one-note wonder. Simon Cowell in particular has grown impatient with what he calls her “screaming,’’ and it remains to be seen whether his judgment costs Magnus a real shot at the “Idol’’ crown.

Parochialism aside, many who know the Magnus family personally said they’d be rooting hard for Siobhan even if she weren’t so darned talented.

“They’re an amazing family, very down to earth,’’ said Kevin Richards before Tuesday’s telecast. Richards, a neighbor and family friend, reported that his contracting work takes him to the Boston area regularly, and everywhere he’s traveled lately, from bank to doctor’s office, the chatter has been about Siobhan and her “Idol’’ chances.

Azora Magnus, 8, and her 9-year old sister, Ceili, the youngest of six Magnus children, both said it was exciting to have a big sister turning into a national celebrity, although Ceili conceded she was tiring of the attention she’d been getting at school. Packing the theater around them, meanwhile, were scores more fans who’d come from around Cape Cod to “get our Siobhan on,’’ as one put it.

Paula Lepore of Centerville and Ginger Coakley of Yarmouthport flashed their pro-Siobhan pins while waiting for “Idol’’ to start. Patty McLean of Falmouth sported a T-shirt proclaiming “I Voted for Siobhan.’’ A case manager at Barnstable High School, Magnus’s alma mater, McLean said the school might air the “Idol’’ finale at its giant outdoor theater, should Siobhan get that far. WQRC-FM morning show host Cheryl Park said her station, too, was riding the Siobhan Express as far as it gets, with theatrical broadcasts being planned at Mid Cape and Lower Cape locations in coming weeks.

Sadie Higgins of Wellfleet came dressed up as a “Siobhan-bie,’’ a zombie-like look inspired by Magnus’s known fondness for horror films. Higgins attends Eastham’s Nauset Regional High School and said that having a native Cape Codder vying to become the next American Idol has her school a-buzz as well. “I’ve always been an ‘Idol’ fan,’’ Higgins said, “but this makes it way more exciting.’’

Joseph P. Kahn can be reached at jkahn@globe.com.