WASHINGTON (AP) -- Mitt Romney got two important wins in the Arizona and Michigan primaries, but he better pick up the pace if he wants to lock up the Republican nomination for president before the party's national convention in late August.
Voters in 11 states have gone to polls and Romney has won six of them. Perhaps even more important, he has won 51 percent of the delegates at stake in those contests.
But at that pace, the former Massachusetts governor won't ever win enough delegates in the primaries and caucuses to clinch the nomination. He would need help from Republican National Committee members who automatically attend the convention and can support any candidate they choose.
Even without reaching the 1,144 delegates needed for the nomination, Romney could potentially build an insurmountable lead and become the obvious nominee. But his slim margin for error, inability to build lasting momentum and trouble connecting with the party's conservative base provide incentives for the other three candidates to stay in the race.
Former Pennsylvania Sen. Rick Santorum says it's down to a two-man race, trying to make the case that he's the candidate who can rally the anti-Romney forces within the Republican Party. Santorum has halted Romney's momentum before. After Romney posted big wins in Florida and Nevada, Santorum upended the race by sweeping the Feb. 7 caucuses in Colorado and Minnesota and nonbinding primary Missouri.
"A month ago they didn't know who we are, but they do now," Santorum said Tuesday night after narrowly losing to Romney in Michigan.
Romney and his aides repeatedly have said their campaign is built for the long haul, with more money and a more extensive organization than his rivals.
"On to the March contests," Romney said, looking ahead to voting in 10 states next week on Super Tuesday.
In race for delegates, Romney leads with 165, including endorsements from RNC members. Santorum has 85 delegates, former House Speaker Newt Gingrich has 32 and Texas Rep. Ron Paul has 19.
If Romney and Santorum keep winning delegates at their current pace, Romney's lead would grow to more than 240 delegates by the end of March. By the end of April, his lead would be about 320 delegates.
"What I think happens over the course of March is, one or the other candidates will build such a lead that the world recognizes that that's going to be the guy," said John Ryder, an RNC member from Tennessee who served on the panel that wrote the party's rules for awarding delegates. "There's a point at which it becomes mathematically impossible, or unlikely, for any of the other candidates to overtake the leader."
Some 2,286 delegates are slated to attend the party's national convention in Tampa, Fla.; 2,169 of them are at stake in the primaries and caucuses in each state. The RNC delegates make up the other 117. It takes a majority of the delegates, or 1,144, to win the Republican nomination for president.
So far, the vast majority of RNC members are taking a wait-and-see approach. Romney has endorsements from 18 of them, Gingrich has three and Santorum and Paul have one apiece, according to a survey by The Associated Press.
On Tuesday, Romney picked up 29 delegates in Arizona's winner-take-all primary. In Michigan, he won the statewide vote but he and Santorum were on pace to roughly split the state's 30 delegates because Michigan awarded most of its delegates based on results in individual congressional districts.
Several factors are helping turn the race into a long march. Nearly every state uses some sort of a proportional system to award delegates, so even losing candidates can win a significant number. As long as at least three candidates stay in the race, it will be difficult for one candidate -- even a successful one -- to win much more than half the delegates.
In 2008, 13 states awarded all of their delegates to the statewide winner. This year, only seven states plan to award their delegates winner-take-all, if Idaho is included, with its unique caucus system that the state GOP says will probably result in all of its delegates going to one candidate.
The calendar is also back-loaded, with five primaries scheduled for June 5, including contests in delegate-rich California and New Jersey. Eight primaries are scheduled for May -- nine, if Texas eventually lands in May after resolving its redistricting dispute.
As the race evolves into a hunt for delegates, all the Republican candidates should focus on winning delegates even in states where they will lose the statewide vote, said Jeff Berman, who ran President Barack Obama's delegate operation in 2008. That means targeting friendly congressional districts in some states and local caucuses in others, much like Obama did in his primary battle against Hillary Rodham Clinton.
"With most states adopting some form of proportional allocation of delegates for 2012, the candidates will have to treat this race more like the Democratic contest of 2008, when Obama hunted delegates one by one to build and maintain his narrow delegate lead," said Berman, who recently wrote a book about Obama's 2008 campaign, titled "The Magic Number."
Next week's Super Tuesday contests could go a long way toward defining the rest of the race. A total of 419 delegates are at stake in the 10 states -- more than a third of the delegates needed to win the nomination.
The Super Tuesday contests, however, illustrate the difficulty of amassing large numbers of delegates.
Even if Romney has a fantastic Super Tuesday, winning seven of the 10 states and coming in a strong second in Georgia, Tennessee and Oklahoma, he could get only about 60 percent of the delegates, according to an AP analysis.
A day like that could knock one of the other candidates out of the race. But if it doesn't, it would only slightly improve Romney's delegate math.
Instead of a delegate fight at the convention, Romney would be on pace to clinch the nomination with a victory in the last primary of the season -- Utah, on June 26.