Mitt Romney was leading Rick Santorum in a tight race for the state of Michigan, in a primary contest once seen as a sure victory but now considered critical in the tumultuous Republican race to find a rival for US President Barack Obama.
As expected, Romney easily won the second primary of the last night, in Arizona. He had been the heavy favorite and his opponents had spent little time campaigning there.
But a less-than-clear victory in his native state of Michigan would cast doubt on Romney's status as front-runner and deal a huge blow to his campaign ahead of next week's crucial contests in 10 states, known as Super Tuesday.
Santorum, needs a win or a very close second to show that he is still in play and that victories earlier this month in Minnesota, Colorado and Missouri were not a fluke.
Returns from 50% of Michigan's precincts showed Romney at 40% and Santorum at 36%. Texas Rep. Ron Paul was winning 12% of the vote to 7% for former House Speaker Newt Gingrich.
Romney is viewed as the candidate best positioned to beat Obama in November's presidential election, and he has the backing of much of the Republican establishment. But Santorum has captured the heart of the party's conservative base, which has no doubt about the authenticity of his views on social issues such as abortion and considers Romney too moderate.
"It's very easy to excite the base with incendiary comments," Romney said on Tuesday, in response to a question about why he is struggling to win over Republican voters in Michigan. "I'm not willing to light my hair on fire to try and get support. I am what I am."
Santorum has attacked Romney, son of a former Michigan governor and chief of a now-defunct automaker, for opposing the automobile industry bailout while supporting bailouts for banks. Santorum, a former senator from Pennsylvania, has opposed both.
Romney, a former governor of Massachusetts and ex-CEO of a private equity firm, has campaigned for the most part by emphasizing his business acumen at a time when the US economy is struggling and unemployment remains high. But the lengthening Republican nomination struggle has coincided with a rise in Obama's prospects for a new term.
A survey released during the day showed 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.
The president is unopposed for the Democratic nomination, and timed an appearance before members of the United Auto Workers union in Washington for the same day as the Michigan primary.