Mitt Romney’s weaknesses have been front and center in the Republican presidential campaign. Less discussed but no less important are his opponents’ weaknesses. None of Romney’s rivals has shown the breadth of appeal to be a true threat to win the party’s nomination.

Romney’s victory in the Michigan primary has been widely described as winning ugly. That may be the case, given that Michigan is where he grew up, and he won by three percentage points. What, then, should Rick Santorum’s loss be called? It’s not what Santorum would like the world to believe it is.

The former senator from Pennsylvania hailed his defeat — and what could be an even split in delegates — as something of a victory. Certainly he can take solace in the fact that two months ago he was not even considered a serious candidate and he ended up giving Romney a major scare. But given that he was clearly ahead in the polls two weeks out, the fact that he could not hold that lead says as much about his limitations as it does about his appeal.

That has been the story time and again in the Republican race. This week, it’s Santorum. Earlier, it was Newt Gingrich. He won South Carolina but soon after lost Florida and has been out of the picture since. Ron Paul, for all the talk about his passionate supporters and his organizers’ attention to detail, has yet to win a single primary contest.

The results and the exit polls from many of those states illustrate the problem that Romney’s competitors face as the contest turns to Super Tuesday. The others can win parts of the Republican electorate, or score victories in regional hot spots, but they haven’t demonstrated the kind of broad geographic or demographic support that is required to become a party’s nominee.

Romney has won six of the 11 contests to date. He finished second in four others. His worst finish was third, in Minnesota’s caucuses, behind Santorum and Paul. Santorum has won four contests and finished second in two others. But he has run third four times and fourth once. Gingrich ran second in the two contests that followed his South Carolina victory but faded to third or fourth in all the contests since. Paul has a pair of seconds in two contests in the Northeast, but the rest of his finishes are thirds or fourths.

Romney’s biggest primary defeat came in South Carolina, where he was overrun by Gingrich. Most of his other losses were in states that were not directly awarding delegates. Missouri, for example, was purely a beauty contest. Caucus results in Minnesota and Colorado may or may not affect how the delegates ultimately are apportioned. That doesn’t excuse Romney and his team. Letting Colorado, a state he won in his 2008 presidential bid, get away was one of the campaign’s major miscalculations.

So Gingrich’s lone victory was significant, but he has not translated it into anything since. Santorum couldn’t leverage what first looked like a close second in Iowa — and later became a victory — into any immediate momentum in New Hampshire or South Carolina. His capture of Colorado, Minnesota and Missouri on the same day provided a huge psychological boost but no delegates. Nor, ultimately, did those wins propel him to victory in Arizona or Michigan.

Exit polls tell a similar story. It’s true that Romney has struggled to win the most conservative Republicans in the primaries and caucuses. He lost those voters to Santorum in Iowa and Michigan. He lost them to Gingrich in South Carolina and in Florida. But he captured them in New Hampshire in January and in Arizona on Tuesday.

Romney has won voters who call themselves “somewhat conservative” and those who say they are “moderate” or “liberal” in South Carolina. He may have trouble exciting the base, but his rivals have shown less ability to reach further across the ideological spectrum in their own party.

In Tuesday’s contests, Santorum won among evangelical Christians in Michigan and roughly split them with Romney in Arizona. But he lost badly in both states among non-evangelicals. Gingrich won them in South Carolina and split them with Romney in Florida. But like Santorum, he lost among non-evangelicals in both states.

In South Carolina, 65 percent of primary voters called themselves evangelicals. Most states outside the South will not have such a high percentage of evangelical Christians in the GOP primary electorate. On Tuesday, 42 percent of the voters said they are evangelicals. If Romney wins a large share of those voters, as he often has, and does far better than his rivals among those who are not, that will be a winning formula.

Romney’s rivals have been generally successful in capturing those who strongly support the tea party movement. Santorum won those voters in Michigan and Iowa, while Gingrich won them in South Carolina and Florida. But Romney typically wins among those who somewhat support the tea party.

He has been the consistent favorite among those GOP primary voters who oppose the movement, except in Michigan, where Santorum won among those who strongly support and those who strongly oppose the tea party movement — probably the result of Democratic mischief-makers.

On two core questions that voters are using to evaluate their candidates, Romney has been a steady winner, according to exit polls. In all the contests except South Carolina, he has won among Republicans who say that defeating President Obama is their top priority. In all but South Carolina, he has won among those who say the economy is the most important issue.

Romney’s weaknesses could make this nomination battle longer and more contested than it otherwise might be. Until he begins a real winning streak, questions about him will continue. Even then, because no one can accumulate delegates quickly until a batch of winner-take-all primaries, he can’t reach the magic 1,144 number needed to lock up the nomination.

Santorum showed in Michigan that he can come close to Romney, and his advisers said Wednesday that the allocation of delegates is likely to be an even split in the state. For that they called Michigan a tie. But on Tuesday, Romney won the popular vote in two states and more delegates overall, because he captured all 29 delegates in Arizona while splitting those in Michigan.

Super Tuesday probably will result in some losses by Romney, especially in the South. Gingrich hopes to hold Georgia, the state he represented in the House. Santorum will compete there but will have his best opportunities in Tennessee and Oklahoma. Only Santorum now looks capable of taking on Romney in Ohio, which will be the major media focus next Tuesday.

Those results would serve to extend the race further. But until one of Romney’s rivals begins to show with real consistency an ability to expand his appeal, the former Massachusetts governor, however weakened he may appear at times, will remain the heavy favorite to emerge with the nomination.