Voters in 10 US states are set to pick a Republican presidential candidate, as the rollercoaster race reaches a potentially decisive moment.

美国10个州的选民将选举产生共和党总统候选人，本次大选已经进入关键时刻。

Super Tuesday sees contests in Ohio, Georgia, Massachusetts, Tennessee, Vermont, Virginia, Oklahoma, Idaho, North Dakota and Alaska.

Ohio, too close to call between Mitt¹ Romney and Rick Santorum, will be the most closely watched battleground.

The eventual² nominee³ will challenge Barack Obama in November's election.

Front-runner Mr Romney, a former Massachusetts governor and private equity⁴ tycoon⁵, has been riding a wave of momentum⁶ after four consecutive⁷ poll wins.

'David and Goliath'

But former Pennsylvania Senator Santorum - who bills himself as the true conservative in the race and a candidate who can win over blue-collar voters - is hoping to halt his rival's winning streak⁸（连胜） .

Former House of Representatives Speaker Newt Gingrich, making a do-or-die effort to reboot his flagging campaign, has a commanding opinion poll lead in his home state of Georgia.

Texas Congressman⁹ Ron Paul, often dismissed as a longshot candidate because of his libertarian-leaning views, is aiming to notch¹⁰（赢得） up his first wins in Alaska and North Dakota's caucuses¹¹.

Rust-belt Ohio, a crucial swing state, is rated as Tuesday's most coveted¹² prize as no Republican nominee has ever become president without winning it at the primary stage.

"I hope that I get the support of people here in Ohio tomorrow, and in other states across the country," said Mr Romney on the eve of the poll. "I believe if I do, I'll get the nomination¹³."

His well-funded political machine has far outspent Mr Santorum in Ohio, chopping down his double-digit opinion poll lead in the Midwestern bellwether¹⁴ state.

"To suggest this is David and Goliath is probably an understatement," Mr Santorum told reporters on Monday.

A devout¹⁵ Catholic who opposes abortion¹⁶ and gay marriage, he has sought to exploit lingering（拖延的） doubts among some conservatives about Mr Romney, a Mormon who governed in a liberal state.

As well as Ohio, Mr Santorum is also hoping for victories in Oklahoma and Tennessee, although polls show Mr Romney gaining on him in the latter state.

Mr Romney is seen as the favourite in Virginia's primary, where Mr Santorum and Mr Gingrich failed to qualify for the ballot¹⁷, and as the all but certain victor in his political heartland of Massachusetts.

He is also expected to win in Idaho, a state with a large Mormon population, and Vermont.

Mr Romney has picked up several coveted endorsements¹⁸ in recent days, including Republican House Majority Leader Eric Cantor, former attorney general John Ashcroft and Oklahoma Senator Tom Coburn.

Out of the 1,144 delegates needed to secure the Republican presidential nomination at the party convention（政党代表大会） in August, more than 400 are at stake in Tuesday's contests.

Ahead of the 10-state voting marathon, Mr Romney had a solid lead with 203 delegates, compared with 92 for Mr Santorum, 33 for Mr Gingrich 33 and 25 for Mr Paul, according to the Associated Press.