When a politician messes up, there is often a political guru standing ready to clean things up. But what happens when the guru makes a mess of his own?

That is the question for Eric Fehrnstrom, one of the most senior advisers to Mitt Romney. Part enforcer, part alter ego and often grim, Fehrnstrom is usually the one who is vigilant about staying on message.

But the morning after Romney’s big win in the Illinois primary, Fehrnstrom blunted the Republican presidential front-runner’s momentum by saying on CNN that Romney would “hit a reset button” in the general election.

“It’s almost like an Etch a Sketch,” Fehrnstrom said Wednesday. “You can kind of shake it up, and we start all over again.”

Romney’s political opponents pounced. Etch a Sketch went viral. Suddenly the image of a children’s toy erasing drawings with a simple shake reignited concerns about the certainty of Romney’s conservatism.

The message didn’t help Romney, nor did the messenger. Although Fehrnstrom is one of Romney’s longest-serving aides, he, like his boss, has no roots in the conservative movement. Fehrnstrom started out as a reporter covering alderman meetings in Boston and lives in liberal Brookline, Mass. His work as a GOP operative has been confined to New England — a region, as he often notes, that’s “pretty rocky terrain for Republicans.”

To some influential conservatives, Fehrnstrom is an enigma. It’s not that conservatives don’t trust him. It’s that some don’t even know him.

“I’ve never heard of him. I’ve been going to conservative meetings since I was knee-high to a grasshopper, and I’ve never seen him anywhere,” said Craig Shirley, an adviser to conservative organizations who has worked for Newt Gingrich, one of Romney’s opponents.

Despite the gaffe, it appears unlikely that Fehrnstrom will be demoted or will lose his job. He is one of Romney’s most trusted advisers — if he worked for President Obama, he would be a cross between David Axelrod and Robert Gibbs — and Romney is fiercely protective of his loyalists.

“Eric is the first guy to jump out there and defend Mitt,” said Romney’s communications director, Gail Gitcho. She added that Fehrnstrom, who runs senior staff briefing calls with Romney every morning, provides “very careful counsel.”

Inside Romney’s Boston headquarters, Fehrnstrom is regarded for his determination and steadiness — “he just moves through the water,” senior adviser Beth Myers said — as well as his encyclopedic memory about all things Romney.

This week, when Axelrod told CBS News that he admires the Romney campaign because it has been “doggedly tenacious,” Romney aides believed he was referring to Fehrnstrom.

“He’s also got a great intuition for seeing around the corners,” Myers said. “He has a good understanding of human nature and good intuition. And he’s also just whip-smart, which people may not know about him because he’s not braggy.”

Fehrnstrom, 50, entered politics in 1994 when Joe Malone, then the state treasurer, plucked him from the Boston Herald, where he had been covering the State House, to be his communications director.

“He is unwavering in terms of his mental toughness,” Malone said. “He’s got, in the best sense of the word, a warrior’s mentality.”

In 2002, while Fehrnstrom was working at an ad agency, Romney confidant Bob White sought him out to run communications on Romney’s gubernatorial campaign. After four years with Romney at the State House, Fehrnstrom became his spokesman and senior traveling aide on the 2008 presidential campaign.

Aside from a testy exchange with Boston Globe reporter Glen Johnson that earned him the moniker “Mitt’s Pit Bull,” Fehrnstrom kept a low profile.

“It’s unusual for him to be that sloppy and that undisciplined,” Jeffrey Berry, a professor of political science at Tufts University, said of the Etch a Sketch comment. “I’ve known nothing similar in his history in Massachusetts.”

After the 2008 campaign, Fehrnstrom, Myers and a third Romney adviser, Peter Flaherty, started a consulting firm, Shawmut Group. They were the architects of Scott Brown’s 2010 campaign for Edward M. Kennedy’s Senate seat.

This year, Fehrnstrom is juggling work for Brown and Romney, helping both candidates craft often divergent messages. The former is embracing the Massachusetts-moderate label; the latter is running away from it.

For instance, after Rush Limbaugh recently used a slur to describe a Georgetown University law student who had testified in support of birth control, Brown tweeted that his comments were “reprehensible” and called on the radio talker to apologize. Romney, meanwhile, said only that Limbaugh’s was “not the language I would have used.”

Fehrnstrom’s work for Brown got him in some hot water in the summer, as he used a fake Twitter identity, @CrazyKhazei, to fire off snarky quips about one of Brown’s Democratic opponents, Alan Khazei. His hand was exposed when he accidentally sent one of his nastygrams under his own Twitter handle, @EricFehrn.

On Wednesday, when Fehrnstrom spoke with Romney about his Etch a Sketch comment, Romney is said to have taken it in stride. Campaign officials circled the wagons and insisted Fehrnstrom had not erred. Privately, some expressed shock at the media attention Fehrnstrom’s remark drew.

On Thursday, Fehrnstrom brushed off his gaffe with humor. After Ohio Art Co., the maker of Etch a Sketch, reported that shares soared, Fehrnstrom tweeted: “Etch A Sketch stock is up? Psst, I’ll mention Mr. Potato Head next. Buy Hasbro.”

In an interview, Fehrnstrom brought up Vice President Biden’s frequent slips of the tongue.

“I thought it was pretty clear with that Etch a Sketch comment I was referring to the race, and not the candidate, but such is politics,” he said. “I’m expecting a call from Joe Biden thanking me for taking the heat off him for a couple of days.”