Rick Santorum’s trio of victories in Colorado, Minnesota and Missouri on Tuesday is shifting the Republican presidential contest into a state-by-state fight for delegates that could last much longer than initially anticipated.

The battle appears to be breaking down along regional lines, with Santorum gaining momentum in the Midwest, Newt Gingrich resonating in the South and Mitt Romney faring best in the Northeast and elsewhere. Those divisions could become sharper as the race goes on, and as more states in each region participate.

For now, all four candidates are on a hunt for delegates, each with an incentive to stay in through the next several contests. Gingrich and Ron Paul were already pledging to stay in the race for the long haul, and both have shown an ability to win delegates. While Santorum’s campaign had been struggling, he now appears to be in contention for at least the next month, including Super Tuesday on March 6.

And all four candidates will compete in the 20 contests held March 6 or before — at least.

In addition, delegates will be awarded on a proportional basis in most of the early states — as opposed to on a winner-take-all basis — which means several candidates can earn delegates in each state, which will in turn make it harder for any single candidate to build a big delegate lead. AP projections based on Tuesday’s results have Santorum claiming second place in the delegate race; if he fares well in the next two contests, in Arizona and Michigan on Feb. 28, he could significantly shrink Romney’s lead by Super Tuesday.

Momentum so far has been elusive. Few candidates have gotten a bounce from one contest to the next. That makes it harder to predict the future outcomes based on past results.

“I think the results leave us with a scattered field,” said Republican National Committeeman for Tennessee John Ryder. “We are past the ‘battle of annihilation.’ Nobody has been knocked out. All four remaining candidates have the ability to raise money and continue their fight.”

As they proceed, the candidates appear to be gravitating toward their regional ties.

Santorum has won three Midwestern states, including two by huge margins, and Gingrich seems to be reverting to a kind of Southern strategy that relies heavily on those states south of the Mason-Dixon Line. His lone win came in South Carolina.

To win the nomination, a candidate will need a majority of the 2,286 delegates at stake. If those delegates are being split four ways, it becomes much harder for one candidate to reach a majority — 1,144 delegates. If no candidate wins that many delegates, the nominee would be decided at the party’s August convention.

If the race does become a delegate battle, here’s how it might work:

Feb. 28 — Arizona and Michigan primaries (59 delegates)

Arizona’s primary is winner-take-all for its 29 delegates, while Michigan will award its 30 delegates on a more proportional basis.

Romney has decent prospects in both these states: Arizona has a significant Mormon population, and his father was governor of Michigan. But he has also been hammered over his position against the auto bailout, which could hurt in the Wolverine State.

Of the two states, Michigan appears poised for a bigger showdown. It could pit Romney’s inherent strengths against Santorum’s blue-collar Midwestern appeal. But the real delegate prize is Arizona, so the smart money will be spent there if the campaigns are preparing for a delegate race.

March 6 — Super Tuesday (437 delegates)

Ten states will hold GOP contests this day; there will be primaries in Georgia, Massachusetts, Ohio, Oklahoma, Tennessee, Vermont and Virginia, and caucuses in Alaska, Idaho and North Dakota. Essentially all of these states have some kind of proportional system of allocating delegates.

Gingrich figures to do well in Southern states such as Georgia and Tennessee, but he failed to qualify for the ballot in a third, Virginia. Romney will almost certainly do well in his home state of Massachusetts and probably in Vermont, too. Santorum may connect with blue-collar workers in Ohio. And Paul has some caucus states where he can steal some delegates; he took at least 17 percent of the vote in 2008 an all three of these caucus states.

The key battles here are most likely to be in Ohio, where most of the 66 delegates are available, three at a time, to the winner of each congressional district. That could be a big swing if a candidate sweeps the state, and it’s why Gingrich and Romney are already focusing on Ohio.

But also look at Oklahoma, which is kind of a hybrid of the Midwest (Santorum’s strength) and the South (Gingrich’s strength).

In other words, there are constituencies here for everyone to win, and it could be close if all the candidates are still seen as viable.

The rest of March (430 delegates)

Eleven more contests will be held in March. The big ones are primaries in Alabama, Illinois, Louisiana and Mississippi, along with key caucuses in Kansas and Missouri.

This may be Romney’s toughest stretch, as most of these states are either Southern or Midwestern. But he should have an advantage in the biggest state, Illinois (69 delegates), both because it’s a more moderate state (despite being Midwestern) and because of a complicated delegate-selection process that favors an organized campaign.

April (484 delegates)

This is where a delegate race would start getting decided, because it is when every state can start allotting delegates on a winner-take-all basis.

Yet the biggest April states — Texas and New York (250 delegates combined) — are still proportional, which means the race could continue even after April if those states are split up sufficiently.

Gingrich is counting on Texas, but, as always, it’s too simple to just label it a Southern state. New York would obviously seem to be Romney territory, but again it’s not clear-cut. Santorum, meanwhile, should do well in his home state of Pennsylvania and Midwestern Wisconsin.

May (276 delegates)

Seven contests this month are highlighted by the Indiana and North Carolina primaries on May 8, but this month includes no pure winner-take-all contests.

The regional advantages at this stage are harder to mete out, but there are some Southern states (Arkansas, Kentucky and North Carolina) and some Midwestern states (Nebraska) for Gingrich and Santorum to play in. Santorum, though, did not qualify for the Indiana ballot.

June (339 delegates)

This is really when winner-take-all starts to take hold. California is the big one (172 delegates), but there are also 90 delegates at stake in winner-take-all contests in New Jersey and Utah. Both New Jersey and Utah are pretty clearly Romney country, so it’s a nice backstop for him if he winds up in a real delegate scrap.

If the race reached this point, it would almost certainly become all about California on June 5.