Mitt Romney has scored a double victory in the latest US presidential Republican primaries, but only after a tight race with main rival Rick Santorum in Michigan.
Romney's narrow win in his native Michigan, however, will do little to dispel doubts about his ability to rally the party's conservative base and take the US presidency from incumbent Barack Obama in November's election.
"Wow! What a night," an obviously relieved Romney told cheering supporters at his state campaign headquarters in Novi, Michigan late on Tuesday night. "We didn't win by a lot, but we won by enough and that's all that counts."
Santorum, who is still riding high on momentum gained from primary wins in Missouri, Minnesota and Colorado on February 7, also highlighted the closeness of the race.
"A month ago they didn't know who we are but they do now," Santorum told supporters after the results were announced. "We came into the backyard of one of my opponents in a race that everyone said, well, just ignore it, you have really no chance here," Santorum said.
With 87 per cent of Michigan's precincts reporting, Romney had 41 per cent to Santorum's 38 per cent. Ron Paul, the Texas congressman, was in third place with 12 per cent, while Newt Gingrich, the former house speaker, had seven per cent.
Comfortable Arizona win
Romney, the long-time frontrunner in the race to challenge Obama, was a more comfortable winner in Arizona, where he was ahead of Santorum by a margin of 48 per cent to 26 per cent, with 62 per cent of precincts reporting. Gingrich was in third with 16 per cent and Paul came in last with eight per cent.
Victory for Romney gives him the votes of all 29 of Arizona's delegates in the race for the nomination. Michigan's 30 delegates will be distributed proportionally.
The vote in Michigan, had threatened to turn into an embarrassing defeat for Romney, who was born and raised there and is the son of a popular governor of the state, in the face of Santorum's re-energised campaign that has transformed him into the standard bearer for the Republicans' socially conservative religious right.
Al Jazeera's John Hendren, reporting live from Detroit, said Santorum's campaign had connected with many voters in Michigan and that even a second place finish could keep the former Pennsylvania senator in the contest.
In a state with over nine per cent unemployment Romney's wealth and referencing of luxury Cadillac automobiles had made him a hard sell to voters, Hendren said.
The candidates now look forward to "Super Tuesday" on March 6, when 10 states hold nominating contests.
Santorum had made himself competitive in Michigan by pressing his conservative views on social issues and by spreading a blue-collar message about the need to rebuild the manufacturing base in the hard-hit Midwestern state.
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An unpredictable factor in Michigan was the ability of Democrats to vote in the Republican primary and try to thwart Romney by voting for Santorum, who many see as having little chance of defeating Obama in the November 6 election should he become the Republican nominee.
The Santorum campaign tried to encourage the crossover vote with a "robocall", urging Democrats to send a message to Romney because of his opposition to 2009 auto bailouts that kept thousands of Michigan workers employed.
The effort was quickly condemned by the Romney campaign as a sign that Santorum was "now willing to wear the other team's jersey if he thinks it will get him more votes".
Romney has been hammering home his view that his experience as a private equity executive and former state governor makes him the best candidate to defeat Obama and lead the US economy back to strong job growth.
All four remaining candidates in the race for the Republican nomination have vowed not to drop out until the Republican national convention in late August, where a result might have to be brokered behind the scenes if no one reaches the magic number of 1,144 delegates.