Mitt Romney secures wins in Arizona and Michigan
US Republican White House contender Mitt Romney has pulled off a double win in the Michigan and Arizona primaries.
"We didn't win by a lot, but we won by enough and that's what counts," Mr Romney said in Michigan.
Michigan was seen as vital for Mr Romney, who was born in the state, but he had struggled to win over conservative voters.
The focus now turns to Super Tuesday next week when 10 states will hold primaries or caucuses.
Those contests could prove decisive in naming a Republican winner to take on President Barack Obama in November.
Former House Speaker Newt Gingrich and Texas Congressman Ron Paul did not actively campaign in Arizona and Michigan, focusing on contests scheduled for next week.
In Arizona, Mr Romney had 47% of the votes to Mr Santorum's 27%; and was ahead in Michigan by 41% to 38%.
Mr Gingrich was on course to poll 16% in Arizona, with Mr Paul on 8%. In Michigan, Mr Paul has won 12% of the vote, with 7% for Mr Gingrich.
Proclaiming his win in Novi, Michigan, Mr Romney told supporters he would hammer home a consistent campaign message in the coming weeks.
"We need more jobs, less debt and smaller government. We've got to hear that day-in and day-out," he said.
Mr Romney - whose father served as Michigan's governor - also criticised Mr Obama for his economic policy and argued that the US "needs a recovery from this recovery".
"What we can't afford is four more years of Barack Obama with nothing to answer to."
Moments after he called Mr Romney to concede victory, Mr Santorum thanked his voters for their backing.
He told supporters he came into "the backyard of his opponent" and did better than expected.
Mr Santorum used his speech to say that as president, he would reclaim manufacturing jobs, cut the corporate tax rate to zero and "repeal every single one of Barack Obama's big government regulations on day one".
Mr Romney will be awarded all of Arizona's 29 presidential nominating delegates, but will share Michigan's 30 delegates, as a district-based system is in place in the Wolverine State.
The Arizona win comes after a barely contested race there, with campaign resources mainly deployed elsewhere in recent weeks.
That contrasts with an expensive, hard-fought campaign in Michigan, where Mr Romney and an independent committee supporting him spent almost $4m (£2.5m) in advertising.
On Tuesday, Mr Romney appeared to acknowledge that he has had trouble winning over conservative voters in a state where he was expected to do well.
He said his disconnect with the party's right-wing stemmed from his unwillingness to make "incendiary" comments.
Mr Romney is a multimillionnaire former governor of Massachusetts who has made much of his business background on his well-funded campaign trail.
Mr Santorum - a Catholic whose staunch social conservatism has made him a favourite of the Tea Party movement - has less financial backing but has made a surprise surge in recent primaries.
In the end, exit polling in both Michigan and Arizona showed that about half of voters "strongly" backed the candidate they voted for.
In Michigan, the state's primary rules allow non-Republicans to register as party members and vote in the primary - meaning that around 10% of all voters identified themselves as Democrats.
Tuesday's two primaries were the first test for Mr Santorum since a three-state sweep in votes earlier in February.
Mr Romney won previous contests in New Hampshire and Florida, but had struggled in recent weeks.
Following Tuesday's votes, the former Massachusetts governor has 163 delegates, compared to 83 for Mr Santorum, 32 for Mr Gingrich and 19 for Mr Paul.
It takes 1,144 to win the nomination at the Republican Party convention this August.