(Image Credit: Gerald Herbert/AP Photo)

When the four remaining Republican presidential candidates gathered in Arizona last Wednesday night, the notion that it could be the last debate of the GOP primary seemed far-fetched. Mitt Romney has seemed anything but inevitable. His campaign warchest and growing list of endorsements - Arizona Gov. Jan Brewer backed him over the weekend - is outweighed by missteps.

A speech to a nearly empty Ford Field in Detroit made enthusiasm for the former Massachusetts governor seem lacking. And his status as a wealth has at times made him seem out of touch.

For instance, Romney told the AP Sunday that he is not a big fan of NASCAR, but he does count several team owners among his friends.

Two candidates - Mitt Romney and Rick Santorum - have won the exact same number of states to date. Clearly, no one is running away with this thing. In fact, the buzz on the campaign trail lately has been more about a brokered convention or a new candidate altogether than the party's coalescing around an established nominee in the coming weeks.

But a closer look at the road ahead suggests that the Arizona debate could in fact be the last one, that a March 19 debate in Portland, Ore., could be wholly unnecessary. Between now and then, starting with primaries in Arizona and Michigan tomorrow, 18 states will hold primaries or caucuses, with Super Tuesday - the much-anticipated March 6 frenzy when 10 states vote - right smack in the middle of it all.

It is unlikely that Romney could wrap up the race in that timeframe, but it is possible, according to Craig Robinson of The Iowa Republican.

"For the race to be essentially over after Super Tuesday, Romney would have to dominate his opponents for the next two weeks," Robinson said. "I don't think that's very likely, but it could happen."

Start with Romney's recent boost. Two weeks ago it appeared that Romney's status as the favorite to win the GOP nod had grown shaky. Earlier this month Santorum had swept three states - Minnesota, Colorado and Missouri - that Romney won in 2008. Santorum had snatched the lead in polls in Michigan, where Romney was born and where his father served as governor for six years. Santorum had tightened the race in Arizona. And Santorum had opened up a double-digit lead in Gallup's national tracking poll.

Then the Romney machine duly went to work, setting its sights squarely on Santorum. In Michigan, Romney and his allies outspent Santorum by a wide margin. At last week's debate in Arizona, Romney attacked Santorum time and time again, putting Santorum on the defensive all night long. It was a not-ready-for-prime-time performance from the Pennsylvania senator, highlighted by his admission that at one point during his tenure on Capitol Hill he had voted "against his principles."

What had once been seen as a golden opportunity for Santorum to seize control of the primary suddenly became a damaging blow to his upstart campaign. Polls in Michigan now show Romney in the lead. Only two weeks ago two polls - one from Public Policy Polling and one from American Research Group - had put Santorum up between 6 percentage points and 15 percentage points over Romney, but a Rasmussen poll released last Friday revealed Romney now up by six percent.

"We are going to win Michigan," Romney senior advisor Stuart Stevens confidently proclaimed after the debate.

Coupled with Romney's lead in Arizona - an NBC/Marist poll this week put Romney up 43 percent to 27 percent over Santorum - tomorrow night is shaping up to be a very good one for the former Massachusetts governor. Traders at the online exchange Intrade on Friday gave Romney nearly an 80 percent chance of winning his party's nomination.

Assuming Romney emerges victorious in both of tomorrow's contests, he will have vanquished the man who appears to be the last remaining obstacle between him and the nomination. Ron Paul has yet to win a single state. Newt Gingrich won South Carolina, but slumped to a disastrous defeat in Florida after Romney and his allies pummeled him with a relentless barrage of negative attack ads.

Now it's Santorum's turn. And so far, it seems, the glare of the spotlight is taking a toll. In addition to his dismal debate performance, Santorum has had to explain away a speech he gave in 2008 at Ave Maria University in Florida when he said Satan is targeting the United States. Not what a presidential hopeful wants to be talking about at a crucial time in the primary.

If Romney wins Arizona and Michigan - as well as the Washington state caucus, where he is trailing Santorum - he will have all the momentum heading into Super Tuesday on March 6. He is already a virtual shoo-in to win a handful of states that day: Massachusetts (where he governed), neighboring Vermont, and Virginia (where Santorum and Gingrich are not on the ballot). In addition, he is favored in Idaho, where there is a strong Mormon population. That puts Romney in an enviable position even before the focus turns to the four states seen as the day's big prizes: Georgia, Ohio, Tennessee and Oklahoma, with a combined 243 delegates between them. According to the latest polls, Gingrich has a slim lead in his home state of Georgia, while Santorum has the upper hand in Ohio and Tennessee.

But if Romney wins in just one of those four places, that would give him victories in half of the Super Tuesday states, with the remaining states likely split between his three rivals. A win in Ohio would be especially important. Two of those rivals - Gingrich and Santorum - would each have seen their best shots slip away, the former in Florida and the latter in Michigan. And Romney's organizational strength, his hefty war-chest and his relentless attacks would leave them all feeling defeated, even if mathematically they still had a chance. After all, the effects of Romney's attacks can leave a lasting mark: Gingrich, according to the latest ABC News/Washington Post poll, has dropped to 45 percent favorability within the Republican party. By comparison, Romney is at 65 percent.

"The only way that Romney can become the inevitable nominee is if he stops losing," Robinson said. "I don't think it's going to be easy for him in Ohio, Oklahoma or Tennessee. He has to win a Southern state to essentially kill Santorum and Gingrich."

However, an alternative scenario is that Romney could emerge from the next few weeks to find himself locked in a two-man race with Santorum and nursing some harsh wounds going forward. For instance, Robinson noted, a loss to Santorum in Ohio on Super Tuesday would "severely wound" Romney, while losses in Georgia and Tennessee could create "a narrative that Romney can't win Southern states [that] would be very damaging."

In other words, politicos - don't book your tickets to Portland just yet, but don't make any other plans either.

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