In the aftermath of Rick Santorum’s clean sweep of Colorado, Minnesota and Missouri, Mitt Romney is still, in fact, the front-runner for the Republican presidential nomination. But the lack of enthusiasm for his candidacy among conservatives foreshadows a potentially ugly road ahead to the party’s convention in Tampa and general-election problems if he becomes the nominee.

Romney advisers can offer all kinds of reasons for his anemic showing Tuesday. Some of them were cited in a memo issued as the polls were opening. The candidate was coming off two consecutive victories that should have given him momentum. But the defensively worded memo was designed to dampen expectations.

The arguments advanced by Romney’s advisers are not incorrect. He did not spend real money in these states. He did not campaign in them much. No delegates were awarded in any of the three. Missouri was simply a beauty contest. Arizona and Michigan, whose primaries will be held at the end of the month, look favorable for Romney. Super Tuesday offers more opportunities to win. Santorum and Newt Gingrich have limited resources.

The campaign memo also said this: “It is difficult to see what Governor Romney’s opponents can do to change the dynamics of the race in February.” In writing that, Romney’s top advisers could not have been anticipating that their candidate would suffer a triple defeat Tuesday.

The issue is not whether Romney has been significantly derailed from his path to the nomination, but rather what kind of nominee he might be and what kind of party would be behind him.

The states that held contests on Tuesday were not inhospitable to the former Massachusetts governor. Four years ago, he won Colorado with about 60 percent of the vote and Minnesota with about 40 percent. He ran third in Missouri, where he received 29 percent of the vote, just four points behind John McCain.

On Tuesday, he won just 35 percent of the vote in Colorado and 17 percent in Minnesota (a third-place finish). In Missouri, he managed to gain 25 percent of the vote to Santorum’s 55 percent, although the 2008 primary awarded delegates and this year’s did not. In raw votes, he was well below his 2008 levels.

All front-runners lose states along the way. Four years ago, McCain certainly did, as did Barack Obama. Obama became the Democratic nominee without winning California, New York, New Jersey, Ohio or Pennsylvania. He lost the Florida contest and didn’t compete in Michigan. He still became president.

So Romney and his advisers can say that they are still on what they call the methodical path toward the nomination, that they are winning delegates whenever delegates are being awarded. That’s all correct.

But that methodical path they describe also could give him the nomination while leaving his party fractured, the base uninspired and his opponents bitter and slow to reconcile. On Tuesday, turnout was below the levels in 2008. Republicans are fervent in their desire to defeat the president in November but can’t work up much enthusiasm for their candidates.

That is especially true of the designated front-runner. One lesson out of the first nine contests is that, when Romney does not have a built-in advantage, he must rely on negative campaigns to win.

Two of his victories came on friendly turf: New Hampshire, where proximity to Massachusetts helped him, and Nevada, where the sizable Mormon population was an asset. On more neutral terrain, he owes much of his success to the power of negative campaigning. He almost won the Iowa caucuses, and came close only after he and the super PAC behind his candidacy spent millions trashing Gingrich with negative ads. Romney’s victory in Florida came after another heavy investment in attack ads aimed at the former speaker.

His advisers argue that in Florida, he was the more effective and compelling candidate. But it wasn’t Romney’s positive message that turned the tide in that state. It was more his strategy of tearing down Gingrich. A Republican strategist, who spoke on the condition of anonymity so he could offer a candid analysis of the race, said of the Romney campaign Wednesday morning that “they have been effective at gaining altitude by disqualifying their opponents.”

Romney has taken down Texas Gov. Rick Perry once (with Perry’s help), twice knocked down Gingrich and now will focus his attacks on Santorum, a former senator from Pennsylvania.

“I’m sure that right now, the Romney campaign bulldozer is slowly turning and grinding its way forward to roll over and crush Rick Santorum with negatives,” said Alex Castellanos, a GOP strategist. “However, Romney is winning tactically, not strategically. He can’t seem to find a way to win the war. Instead, he has been reduced to fighting and winning 50 separate state-by-state battles, with more money, power and negatives instead of message.”

Some Republican strategists argue that Romney remains well positioned for the general election and deserves credit for sticking to his plan for winning the nomination and not caving in more to conservatives. But as one strategist said, also speaking on the condition of anonymity: “The opportunity cost is that our primary voters are not only lacking enthusiasm for him, they flat-out reject him by huge margins when it is a two-man race, as it largely was last night.”

Romney’s good fortune is that he is not in a two-person race. After South Carolina, Gingrich thought the contest would come down to him and Romney. Santorum now will try to become the lone alternative to Romney. For that reason, Gingrich was the other big loser Tuesday night. As strategist Todd Harris put it, “Santorum’s victories will likely end up hurting Newt far more than Mitt.”

Given the turbulence in the Republican race, Romney might look different in another month, if he can roll up the series of victories through Super Tuesday that his campaign expects. But Republicans must be worrying about where this is heading.

Obama and Hillary Rodham Clinton were strengthened by their long Democratic primary contest four years ago. So far, Romney has been weakened by the competition he has faced. Can he find a positive vision that will energize his base and strike a chord in a general election? That must be as much a part of his objective as simply accumulating enough delegates to claim the nomination.

For more Dan Balz columns, go to postpolitics.com.