Mitt Romney is trying to capitalize on twin victories in Arizona and Michigan as the GOP nomination race expands to the 10 states that vote on Super Tuesday. Rival Rick Santorum, who narrowly lost in Michigan, faces splitting the conservative vote with Newt Gingrich as the former House speaker counts on Southern primaries to revive his campaign.
Texas Rep. Ron Paul remains a factor as he attempts to mine delegates in caucus states like North Dakota, where his campaign team says the process plays to his strengths.
Super Tuesday is March 6, just three days after Saturday's Washington state caucuses.
Romney's slim victory in his native Michigan raised questions about whether he would change his strategy. He acknowledged Tuesday that he had made personal mistakes in recent weeks and said he was trying to "do better and work harder."
All four candidates face financial strains as they try to advertise in a series of expensive states to campaign. It would cost about $5 million to run a week's worth of heavy ads across all the states that vote on Super Tuesday.
Romney signaled Tuesday night that he intends to stick to his core campaign message of fixing the economy and reducing unemployment in a nation still recovering from the worst recession in decades.
"More jobs, less debt and smaller government - you're going to hear that" over and over in the states ahead, he said.
Romney scored a hard-won, home-state triumph in Michigan and powered to victory in Arizona, gaining precious momentum over Santorum in the most turbulent Republican presidential race in a generation.
"We didn't win by a lot, but we won by enough," Romney told cheering supporters in Michigan. He also tweeted his delight - and his determination: "I take great pride in my Michigan roots, and am humbled to have received so much support here these past few weeks."
The Super Tuesday races could go a long way toward determining which Republican will take on Democratic President Barack Obama this fall.
Romney will campaign Wednesday in Ohio before he flies to North Dakota. Santorum was already campaigning in Ohio when the verdict came in from Michigan.
"A month ago they didn't know who we are, but they do now," Santorum told his supporters, vowing to stay the conservative course he has set.
Gingrich and Paul made little effort in Michigan or Arizona, pointing instead to next week's collection of contests in all corners of the country.
Gingrich is campaigning Wednesday in Georgia, the state he represented in the House for 20 years. Contests there and in Tennessee give him an opportunity to breathe some life back into his bid. He won in South Carolina but struggled in Florida.
Romney's Arizona triumph came in a race that was scarcely contested, and he pocketed all of the 29 Republican National Convention delegates at stake in the winner-take-all state.
Michigan's primary was as different as it could be - a hard-fought and expensive contest that native-son Romney could ill afford to lose and Santorum made every effort to win.
In Michigan, 30 delegates were apportioned according to the popular vote. Two were set aside for the winner of each of the state's 14 congressional districts. The remaining two delegates were likely to be divided between the top finishers in the statewide vote.
With his victory in Arizona, Romney had 163 delegates, according to the Associated Press count, compared with 83 for Santorum, 32 for Gingrich and 19 for Paul. It takes 1,144 to win the nomination at the convention in Tampa this summer.
The lengthening GOP nomination struggle has coincided with a rise in Obama's prospects for a new term. A survey released Tuesday shows consumer confidence at the highest level in a year, and other polls show an increase in Americans saying they believe the country is on the right track.
Along with the improving economy, the long and increasingly harsh campaign, in which Gingrich and Santorum have challenged Romney as insufficiently conservative, has prompted some GOP officials to express concern about the party's chances of defeating Obama.
If nothing else, the unexpected clash on Romney's home field dramatized that two months into the campaign season - after nearly a dozen primaries and caucuses - the GOP race to pick a nominee remains unpredictable.
Unopposed for renomination, Obama timed a campaign-style appearance before United Auto Workers Union members in Washington for the same day as the Michigan primary. Attacking Republicans, he said assertions that union members profited from a taxpayer-paid rescue of the auto industry in 2008 are a "load of you-know-what."
All the Republicans running for the White House opposed the bailout, but even in the party's Michigan primary a survey of voters leaving polling places showed about 4 in 10 supported it.
Michigan loomed as a key test for Romney as he struggled to reclaim his early standing as front-runner in the race. But Santorum, a former Pennsylvania senator, rolled into the state on the strength of surprising victories on Feb. 7 in caucuses in Minnesota and Colorado and a nonbinding primary in Missouri.
Santorum quickly sought to stitch together the same coalition of conservatives and tea party activists that carried him to a narrow victory in the Iowa caucuses that opened the campaign nearly two months ago.
There are 40 delegates at stake in Washington's caucuses on Saturday, followed by 419 on Super Tuesday, including big primaries in Ohio and Georgia.
The television advertising wars were already under way. Romney and Restore Our Future, the super PAC that supports him, have spent more than $3 million combined on ads in Ohio.
This program aired on February 29, 2012. The audio for this program is not available.