Members of the media gather around an English bull terrier named Petey during a small protest by a group called Dogs Against Romney outside of the Westminster Kennel Club Dog Show on Feb. 14 in New York. (Spencer Platt/GETTY IMAGES)

It happened more than a quarter century ago, at the start of a Romney family summer vacation. But the tale of Seamus, the Irish setter who got sick while riding 12 hours on the roof of Mitt Romney’s faux-wood-paneled station wagon, is ballooning into a narrative of epic proportions.

It has come to characterize the candidate — and not in the favorable way Tagg Romney hoped for when he first talked in 2007 about his family’s annual road trips.

Late-night host David Letterman has been giving the dog near-nightly shout-outs. There are parody Web videos, “Dogs Aren’t Luggage” T-shirts and Facebook groups. (“Dogs Against Romney,” which protested outside last month’s Westminster dog show, has more than 38,000 Facebook fans.) The New Yorker featured a cartoon, with Rick Santorum riding in Romney’s rooftop dog carrier, on its cover last week. In the five years since the story was revealed, New York Times columnist Gail Collins has mentioned Seamus in at least 50 columns.

On Wednesday, the top campaign adviser to Santorum injected the dog into the already bitter Republican presidential race.

In response to a question on CNN about Romney’s criticism of an ad being aired by a super PAC supporting Santorum, John Brabender said, “Quite frankly, I’m not sure I’m going to listen to the value judgment of a guy who strapped his dog to the top of the roof of his car and went hurling down the highway.”

Romney did not simply strap Seamus to the roof. He attached a dog carrier to the roof rack of the family’s Chevrolet station wagon — the “white whale,” as his boys called it — and assembled a shield to protect Seamus from the highway winds.

The Seamus story first surfaced in the Boston Globe in a chapter of a biographical series the newspaper published in 2007, when Romney first ran for president.

One summer day in 1983, as the Globe reported, the overpacked Romney wagon — suitcases, supplies and five sons, ages 13 and under — set off from Boston for the 12-hour trek to his parents’ cottage in Ontario on the Canadian shores of Lake Huron. Romney, then a 36-year-old management consultant, had planned a single stop to refill the tank, get food and go to the bathroom.

Until the evidence of Seamus’s sickness started dripping down the back window.

“Dad!” Tagg, the eldest son, yelled from the back of the wagon. “Gross!”

Romney pulled off the highway, washed down Seamus and the car at a service station, then got back on the highway.

The reactions

To some animal lovers, the story is an example of cruelty.

“It really says this guy is not like us and is mean,” said Scott Crider, an Alabama marketing guru who founded Dogs Against Romney.

Idaho Gov. C.L. “Butch” Otter, a Romney supporter, said he did not know the story about Seamus but did not see any harm in an open-air ride.

“We carry our dogs around in the back of our pickup all the time,” Otter said. “I’ve got two dogs, and I just say, ‘Get in,’ and they jump in the back.”

Romney’s political opponents sense an opportunity. Former House speaker Newt Gingrich raised the incident a few months ago with a campaign video attacking Romney.

President Obama’s reelection campaign has signaled it would use the story against Romney as well. In January, Obama strategist David Axelrod tweeted a picture of Obama holding his dog, Bo, in the presidential limousine and wrote, “How loving owners transport their dogs.”

Political strategists in both parties say this is the rare anecdote that could break through the political chatter and permeate the popular culture.

“It’s a signifier,” Democratic strategist Chris Lehane said. “There are certain events that happen over the course of someone’s life that play into a larger story line and feed into a caricature. Seamus the dog story just plays into a negative story line about a guy who you may not completely trust.”

Douglas Gross, who chaired Romney’s 2008 Iowa campaign and has become a sometimes-critic of the former Massachusetts governor, said that when the story surfaced in the summer of 2007, “people scrunched their noses and said, ‘What?’ ”

“It’s like the height of the Michigan trees,” Gross said, referring to Romney’s recent comments about the trees there being the “right height.” “It’s another one of those things about Mitt that seems otherworldly. It seems abnormal and raises questions about who he is and whether he’s one of us.”

Romney’s aides are sensitive to the Seamus story. Inside campaign headquarters in Boston, a reporter was teasingly admonished recently for broaching the subject, and a campaign spokeswoman did not respond Wednesday to questions about the dog.

The candidate has said little about it. When the Wall Street Journal asked him about his dog in December, Romney was at a loss for words: “Uh . . . Love my dog.”

In 2007, Chris Wallace asked Romney, “What were you thinking?” He told the Fox News anchor: “This is a completely airtight kennel, mounted on the top of our car. He climbed in there regularly, enjoyed himself.”

‘A pure negative’

One Romney adviser said that the Seamus anecdote is “a pure negative” but that the campaign does not see it as overly damaging in a contest they believe is being shaped overwhelmingly by the economy.

“For crying out loud, with 8.3 percent unemployment, if the dog defeats you, you deserve to be defeated,” said the adviser, who requested anonymity to speak candidly. “Come on. You’ve got to run a good campaign to overcome that. President Obama overcame Reverend Wright and Bill Ayers.”

Some Romney supporters played down talk of Seamus.

“Mitt Romney doesn’t have many skeletons, so the fact that the media’s got to go to a dog story bodes well for my candidate,” Rep. Jason Chaffetz (R-Utah) said.

Others voiced disgust that it had become an issue at all.

“To focus on Governor Romney’s dog, Seamus, is shameful,” said Jeb Bradley, a former GOP congressman from New Hampshire. Bradley recalled talking about dogs with Romney as they drove between town hall meetings this past summer. “He was telling me about helping another dog he had deliver puppies. . . . I walked away from it knowing that Governor Romney is a dog lover.”

A few weeks after the story first came out, Romney’s sister Jane told the Globe that Seamus often left the family’s home to visit his “dog friends” in the neighborhood. “He kept ending up at the pound,” she said.

So in the mid-1980s, Romney sent Seamus to live with Jane in California, where he had more space to roam freely. If Seamus did not like riding in the rooftop carrier, she said, he would have found a way out of the space.

“He was a Houdini,” she said.

Staff researcher Lucy Shackelford and research editor Alice Crites contributed to this report.