With his latest string of primary wins, Mitt Romney took another big step in his seemingly unstoppable march to the Republican presidential nomination. But his victories — and the way Romney achieved them — have taken a toll that could do lasting harm as he turns to the general election campaign against President Obama.

In state after state, Romney has grown less popular the longer the campaign wears on and the better voters get to know him. The same thing happened in 2008, the first time Romney sought the GOP nomination.

The former Massachusetts governor remains competitive with Obama even though the president is viewed much more positively, according to polls. But Romney’s image problem heightens the already formidable task he faces in November, trying to dislodge an incumbent spared the costly and divisive intraparty battle that Republicans have waged.

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Romney’s performance Tuesday in Wisconsin was instructive, as his 7-percentage-point victory over Rick Santorum followed a now-familiar pattern.

He started out trailing the former Pennsylvania senator, according to Wisconsin polls. But Romney pulled ahead after vastly outspending his rival, with the help of deep-pocketed allies, strafing Santorum with a ceaseless barrage of negative advertisements — the same thing Romney did to opponents in Iowa, Florida, Michigan, Ohio and Illinois.

Once more, he stumbled en route to victory, committing another in his series of gaffes. Dialing into a “tele-town hall” last week with Wisconsin voters, Romney told what he called a “humorous” story about his father, a former head of American Motors, shutting down a Michigan auto plant and moving production to Wisconsin.

Taken together, the results are striking. In Ohio, perhaps the single most important fall battleground, a recent Quinnipiac University poll found that Romney was viewed more unfavorably after the March primary than favorably, a reversal of his standing as recently as mid-January.

The same is true elsewhere. Michigan, where Romney was born, once offered a prime opportunity to flip a state that went for Obama four years ago. But after hosting one of the most bruising contests of the Republican race — a knockdown that Romney won in a squeaker — Michigan seems to have reverted safely back to the Democratic column.

The damage extends beyond battleground states. Obama has gained considerable ground against Romney in head-to-head matchups nationally, pulling into a modest lead thanks to his greatly improved standing among independent voters, the swing group that is vital to winning the White House.

A recent ABC News-Washington Post poll found that a record 50% of Americans had an unfavorable view of the GOP front-runner and just 34% had a favorable view, the lowest rating for any leading presidential hopeful in decades.

It is hardly surprising that Romney’s image has suffered during the bruising Republican primary. Assessing the political damage at this point is somewhat akin to examining a bleeding patient who just arrived in the emergency room. Romney advisors are confident his wounds will heal; as one put it, the harsh views of the likely GOP nominee are a first impression but not necessarily a lasting one.

Romney can take heart from the candidate with the previous low standing at this stage of the presidential race, then-Arkansas Gov. Bill Clinton, who had a 39% approval rating in an ABC/Post poll in March 1992. He trailed independent Ross Perot andPresidentGeorge H.W. Bush in matchups taken as late as summertime.

Clinton’s fortunes began to turn at his July nominating convention, a triumph of political repositioning that coincided with Perot’s temporary exit from the race, and Romney will have a similar opportunity to reintroduce himself should he take center stage at the GOP’s gathering in August in Tampa, Fla.

There are, however, important differences. Clinton’s political weakness — a reputation for slick talk and slippery answers — did not have the same political downside as Romney’s wealth and perceived insensitivity, fair or not, toward those less well-to-do. Clinton had a common-man touch, not to mention extraordinary campaign skills, that Romney conspicuously lacks.

Moreover, because of the way Romney has won — by burying rivals in an avalanche of attack ads — he has sullied his own image; anyone who has ever heaved a mud pie at close range knows that some dirt splatters back.

The trick for Romney in the general election will be to take the fight to the president without further harming himself.

“I don’t think the American electorate is very much in the mood for harsh talk right now,” said Don Sipple, a veteran GOP strategist watching the primary fight from the sidelines. “People know what it is they don’t like about Obama. They need to know what they would get with Romney, how it would be different and how it would make their lives better.”

Wisconsin offered some bright notes for Romney. Exit polls showed inroads with evangelical Christians and strongly conservative Republicans, two groups that have been among the most resistant to Romney’s candidacy.

But, again, there were trouble signs. His victory margin, 44% to 37%, was unimpressive given Romney’s massive financial advantage, the support of virtually the entire state political establishment and, not least, his seeming inevitability.

Rhodes Cook, a nonpartisan campaign analyst, has compiled statistics showing that Romney’s share of the Republican primary vote so far is a mere 39%. While it may improve, it is far lower than the final showing for any GOP candidate since the modern presidential nominating system was adopted in the 1970s.

Other candidates have won the White House after winning less than a majority of primary votes — Democrats Jimmy Carter and Obama — but both grew more popular as their campaigns progressed. The only Republican with more tepid support was John McCain in 2008.

Perhaps the best news for the GOP’s nominee-in-waiting is that November is seven months away and the worst of the nominating fight finally seems behind him.

mark.barabak@latimes.com