Mitt Romney notched early wins as he fought to establish his dominance in the race for the Republican U.S. presidential nomination on Tuesday but looked unlikely to score a knockout blow that would force any of his rivals from the contest.
Rick Santorum scored convincing wins in Oklahoma and Tennessee, while Ohio, the biggest prize of the night, was too close to call.
Romney won as expected in Virginia, Vermont and his home state of Massachusetts as 10 states held contests on Super Tuesday, the biggest day so far in the roller coaster Republican campaign.
Newt Gingrich won his home state of Georgia, while results from North Dakota, Idaho and Alaska were expected in the coming hours. More than 400 of the 1,144 delegates needed to win the party's nomination are at stake.
All eyes were on Ohio, a traditional bellwether state that could play an important role in deciding the Republican nominee to challenge Democratic President Barack Obama on November 6.
Exit polls showed that Ohio voters viewed Romney as more likely to defeat Obama, but thought Santorum was more sympathetic to average Americans' concerns.
With 22 percent of the vote counted, Santorum edged ahead but the two were still neck-and-neck.
A victory in Ohio and a good showing elsewhere would make Romney, a former Massachusetts governor, the favourite to win the nomination. Even without an Ohio win, his strength in other states all but ensures he will extend his lead in the delegate count.
This is a process of gathering enough delegates to become the nominee, and I think we're on track to have that happen, Romney said after voting in his home town of Belmont, Massachusetts.
A less impressive showing could prompt renewed doubts about his ability to secure the nomination as Republicans continue the state-by-state battle to pick a nominee at their August convention.
Romney, who built a fortune of at least $200 million (127 million pounds) as a private-equity executive, has struggled to connect with conservatives and blue-collar voters.
He doesn't really know what he stands for, said Santorum supporter Katherine Frenz, 36, of Hilliard, Ohio.
In the last days of the Ohio campaign Romney made a point of trying to attract blue-collar voters, but CNN's exit poll indicated that Santorum received more support among the middle-income voters who make up the bulk of the electorate. Romney topped Santorum among voters with income of more than $100,000, while those who earned less than $30,000 were divided between the two candidates.
Santorum, a former U.S. senator from Pennsylvania, has won the support of religious conservatives thanks to his opposition to gay marriage and his views on other hot-button social issues. His controversial comments about birth control and the role of religion have alienated moderate-leaning voters, and Romney has pelted him with negative ads.
Gingrich's strategy of focusing on southern states did not pay off in Tennessee and Oklahoma, but he vowed to stay in the race after his Georgia win.
There are lots of bunny rabbits to run through, I am the tortoise. I just take one step at a time, Gingrich said.
Ron Paul, a U.S. congressman from Texas known for his libertarian views, hopes to score his first win in Alaska.
In recent presidential campaigns, the Super Tuesday wave of primaries and caucuses has often settled the Republican race. But while this year's contests could establish a clear pecking order, the race is likely to stretch until April or May under new rules designed to attract more voters and boost enthusiasm.
Recent polls indicate the lengthy primary season may actually be alienating voters. An ABC News/Washington Post poll released on Tuesday showed that more voters view the candidates negatively than positively. An NBC News/Wall Street Journal poll on Monday found that 40 percent of voters view the Republican Party less favourably than they did before voting started in January.
(Additional reporting by Sam Youngman in Massachusetts, Lily Kuo and Emily Stephenson in Washington and Colleen Jenkins in Atlanta; Writing by Andy Sullivan; Editing by Doina Chiacu and Vicki Allen)