(Image Credit: Rainier Ehrhardt/AP Photo)

Four years ago, Sen. John McCain used a slew of victories on Super Tuesday to all but clinch the Republican nomination, but even though voters in 10 states will head to the polls Tuesday - and more delegates will be awarded than in the entire Republican primary thus far - don't expect such a decisive outcome this year.

With Super Tuesday 2012 featuring fewer than half as many states as the 2008 version - and all 10 states allocating delegates using some type of proportional system - the day is simply a little less super this time around. And even though Mitt Romney would love to score the type of knock-out blow that McCain delivered to him, the best the former Massachusetts governor can realistically hope for is to land a couple of painful punches that hurt key rival Rick Santorum so badly that he will be able to end the fight for good in the coming weeks.

Nowhere is Romney better poised to strike a serious blow than in Ohio, the most hotly contested of this year's Super Tuesday states. With its importance as a key swing state in the general election, Ohio is vital to both Romney and Santorum's hopes of proving to their party that they are the best candidate to battle President Obama in November. For Romney, the chance to defeat Santorum, who hails from neighboring Pennsylvania, in the Rust Belt stronghold of Ohio is his best shot at finally taking a stranglehold on the GOP nomination.

In the past week, Romney, riding the momentum after his big win last week in Michigan and over the weekend in Washington state, has made up ground on Santorum in polls out of the Buckeye State. A Quinnipiac University poll of likely Republican voters released Monday showed Romney has overtaken Santorum in Ohio by the slimmest of margins: 34 percent to 31 percent. Compare that to a poll taken late last month by the same organization that put Santorum ahead by seven points and it is clear that Romney is making a late surge in the state, embarking on a 10 point swing in about a week.

By now, Romney's rivals should be used to that. The GOP frontrunner has already proved in Florida and Michigan that he knows how to come from behind and that he is at his best when his back is up against the wall. In Florida, Romney thrashed Newt Gingrich after the former House Speaker had won South Carolina. In Michigan, Romney vanquished Santorum in the wake of the former Pennsylvania senator's victories in Colorado and Minnesota.

At a rally Monday in Canton, Romney emphasized to Ohio voters that his focus on the economy not only separates him from rivals like Santorum - who has focused more on social issues - but makes him the best candidate to defeat Obama this fall.

"During this campaign there has been discussion about all sorts of issues," Romney said. "I keep bringing it back to more jobs, less debt and smaller government."

Only days earlier, at an event Friday in Willoughby, Santorum had argued, "We're kidding ourselves if we don't talk about doing things to help and encourage the American family, marriage, fathers taking responsibility for their children, putting networks of support for individuals and families here at the local level."

"You go into the areas of America, you go into the areas of Cleveland where you don't see any dads, what do you see?" he asked. "Do you see freedom? Do you see opportunity? Do you see jobs? Do you see police? Do you see government? Everywhere. That's the reality. It has to be a community effort. It has to be a community effort across this country, but we have to talk about it."

After his setback in Michigan, Santorum needs a strong showing not just in Ohio but also down South if he is to keep alive his slim chances of upsetting Romney in the primary. Other Super Tuesday states such as Oklahoma and Tennessee are likely to prove especially friendly ground for Santorum, who has steadfastly enjoyed the support of more conservative voters. In Tennessee, for instance, a very religious state, Santorum should emerge victorious in part due to the backing of evangelicals. Oklahoma, an extremely conservative state, is expected to be an easy win for him.

In Georgia, however, the story is Newt's last stand. After resounding losses in Florida and Nevada, Gingrich turned his focus to his home state.

"I have to win Georgia, I think, to be credible in this race," he said last Thursday in Atlanta.

Win it he likely will, according to recent polls. In a survey conducted last week by Mason-Dixon Polling & Research, Gingrich enjoyed the support of 38 percent of respondents, with Romney and Santorum far behind with 24 percent and 22 percent, respectively. Whether that is in fact enough to make him credible in the race is another story, though, since he is unlikely to fare well in other Super Tuesday states.

While Georgia, Ohio, Oklahoma and Tennessee - with a combined 243 delegates between them - are the day's biggest prizes, another six states head to the polls on Tuesday and Romney is the heavy favorite in four of them. Romney is a surefire bet to win his home state of Massachusetts, neighboring Vermont, and Virginia where Santorum and Gingrich failed to make it on to the ballot. An NBC News-Marist College poll over the weekend showed Romney with 69 percent backing in Virginia. House Majority Leader Eric Cantor, who hails from the state, endorsed Romney on Sunday. In addition, Romney appears poised for a strong showing in Idaho, where there is a sizable Mormon presence.

Should Romney win those four states as well as Ohio he will have had a good night. But if he can somehow win a southern state, too - like, say, Tennessee - then that would for all intents and purposes end the primary battle, according to McCain, who this time around has endorsed Romney.

"I hope that at that point most of us would declare or believe that it's over and start focusing on the real adversary - that, of course, is winning the November election," McCain told CBS News' Charlie Rose Monday on "This Morning."

Craig Robinson of The Iowa Republican believes that Romney "has to win a southern state to essentially kill Santorum and Gingrich." Even if Romney does so, though, both of Romney's two rivals could continue to plow ahead as long as they are funded by the wealthy backers who have come to their aid in recent months. But even then, if the only thing that stands between Romney and the nomination is a weakened Santorum, a bruised and battered Gingrich, and a winless Ron Paul, then it will simply be a matter of time before the former Massachusetts governor eventually racks up enough delegates to secure the nomination.

For any such talk about locking up the nomination to begin in earnest this week, Romney, it seems, has to win Ohio. To that end, his campaign and his Super PAC, Restore Our Future, has shelled out over $3.5 million on television advertising in the state, a fact lamented Monday by Santorum.

"I am being out-spent 12 to 1 in Ohio," he said in an interview with ABC News. "I mean, David and Goliath - I feel that is not even suitable for the amount of pounding that that Super PAC - I mean, just negative ad after negative ad, 24/7. Yet here we are, hanging in this race, and I just think it shows the real weakness of his candidacy."

"We're winning," Santorum claimed. "Whether we end up with the most votes or not, we're winning."

But making Santorum's task all the more difficult in Ohio is his failure to submit certain paperwork that could put as many as 18 delegates in jeopardy there, more than a quarter of the 66 up for grabs in the state overall.

When all is said and done on Tuesday, even the worst-case result for Romney would ultimately do little to derail his candidacy.

"Even Romney's nightmare scenario - a loss in Ohio to Santorum and getting swept by Gingrich and Santorum in the Southern Super Tuesday states of Georgia, Tennessee, and Oklahoma - would likely still see him walk away with the most delegates," ABC News' Rick Klein wrote in his "Political Insights" column. "Layer on that the proportional delegate allocation system in most states and it becomes clear that Santorum and Gingrich simply can't catch up fast enough to erase Romney's lead. The non-Romney candidates may have persuasive arguments, but they can't win by losing. Romney may not surpass the magic number of 1,144 delegates until May or June. But the point is no other candidate is positioned to even come close to that threshold before Romney does."

"Romney has had his flaws and foibles exposed by this extended primary fight," Klein wrote. "The commitment by Santorum and Gingrich to continue on could further undermine Romney's attempt to excite and unite Republicans behind his candidacy. But Romney is in position to turn his claim on the GOP nomination from a question of if into one of when. A win in Ohio, while not critical, would put more pressure on Romney's rivals to exit gracefully, as the trend lines become clear."

Ultimately, while a knock-out blow for Romney will likely remain elusive on Tuesday, it appears that his eventual primary victory over Santorum, Gingrich, and Paul will only be a question of when, not if.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.