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(CNN) -- Mitt Romney will try to break away from his rivals for the Republican presidential nomination in Super Tuesday contests in 10 states that put 419 delegates up for grabs.
The former Massachusetts governor is coming off three wins last week and a growing lead in the delegate count toward the 1,144 needed to secure the nomination to face President Barack Obama in November.
However, Tuesday is the biggest single day of the primary season, and includes showdowns in several states that will determine the ability of challengers Rick Santorum, Newt Gingrich and Texas Rep. Ron Paul to blunt Romney's momentum toward what many believe will be his inevitable nomination.
So far, Romney has accumulated 207 delegates to 86 for Santorum, 46 for Paul and 39 for Gingrich, according to CNN estimates.
Georgia has the most delegates up for grabs on Tuesday but Ohio, because of its status as a crucial battleground state in the general election, is considered the main prize.
A CNN/ORC International poll released Monday indicates that Ohio is a dead heat between Romney and Santorum, with each grabbing 32% of likely GOP primary voters. Gingrich, the former House speaker, was at 14% and Paul was at 11%.
Surveys released a week earlier suggested Santorum led Romney, but they were conducted before Romney's victories in Arizona and his native Michigan on February 28, followed by winning the Washington state caucuses on Saturday.
Gingrich, meanwhile, made clear he needs to win Georgia -- the state he represented in Congress for two decades -- to keep his campaign alive. The CNN/ORC poll showed him with a solid lead at 47% support among likely GOP primary voters, compared to Romney at 24%, Santorum at 15% and Paul at 9%.
However, other polls indicate his Southern strategy of also winning neighboring Tennessee might be in trouble, with Santorum and Romney vying for the lead there.
With about an hour before the first polls closed in some states, early exit polls out of Ohio and Tennessee showed a hesitant electorate split in crucial contests, according to those who responded to questioners.
In Ohio, a key battleground state that somewhat mirrors the rest of the nation, slightly less than a third of the respondents described themselves as "very conservative," about a third called themselves "somewhat conservative" and slightly more than a third said they were "moderate or liberal."
Ohio respondents were somewhat split on the strength of support for their candidate, with 43% saying they "strongly favor" their candidate, 41% said they "have reservations" and 13% said they disliked the other candidates.
While not an indicator of their final votes, in Tennessee, 44% of respondents said Mitt Romney was the candidate most likely to defeat Obama in the fall, while 23% said Rick Santorum, 21% said Newt Gingrich and 9% said Ron Paul.
A leading GOP strategist thinks if Romney does well across the board on Tuesday night, he could come close to locking up his bid for the nomination.
"Even a come-from-behind win in Ohio won't give Romney the momentum he needs to put this race away, but Romney could seal this deal Tuesday if he takes not only Ohio, but Tennessee," said CNN contributor Alex Castellanos.
"If Romney demonstrates he can win in the South, GOP establishment and conservative voters will rally around him and money for his opponents would begin to dry up," added Castellanos, who was a top media adviser for Romney's 2008 nomination bid but who is not taking sides this cycle. "The real test Tuesday is this: Can Romney win not only in Ohio but in the South?"
Romney's campaign was bolstered by endorsements from leading conservatives this week including House Majority Leader Eric Cantor, R-Virginia, Oklahoma Sen. Tom Coburn and former Attorney General John Ashcroft.
The endorsements indicated a growing push in the Republican Party to show Romney can win the trust of conservatives, despite concerns that he is too moderate.
Thanks to a sweep of contests in Colorado, Minnesota, and Missouri on February 7, Santorum went from a long-shot candidate to a co-frontrunner, but he hasn't had a victory since.
"Simply put, he needs to stop the bleeding after three straight losses by winning several states of his own -- including the big one in Ohio," said Gentry Collins, a former political director for the Republican National Committee and the Republican Governors Association.
It's not just Ohio where Santorum has lost his lead. Polls indicate the same thing has happened in Tennessee.
Santorum and Gingrich are not on the ballot in Virginia because they failed to get enough signatures to qualify, and Santorum's not eligible to win some delegates in Ohio because his campaign failed to file required paperwork in some congressional districts and didn't submit a full list of delegates in others.
"If Santorum wins one or two, but fewer than Romney and the narrative becomes his campaign was too inept to make the Virginia ballot or several districts in Ohio, he's in trouble," said Collins, who ran Romney's 2008 operation but is neutral this time. "Remember, Republicans are hungry for a winner against Obama. If the stench of incompetence sticks to him, he's toast."
Paul has focused his efforts on winning delegates in the caucus states so that he can wield influence at the Republican convention in August.
Besides Ohio, citizens in Cantor's Virginia and Coburn's Oklahoma will participate in primaries, as will voters in Ohio, Georgia, Tennessee, Vermont and Massachusetts, while Idaho, North Dakota and Alaska are holding caucuses.
Here is a state-by-state breakdown:
* Alaska is holding caucuses from 8 p.m. to 12 a.m. ET. There are 24 delegates at stake, to be allocated on a proportional basis.
* Georgia is holding a primary, with polls closing at 7 p.m. ET. Gingrich represented Georgia's 6th congressional district for two decades. There are 76 delegates at stake, allocated on a proportional basis.
* Idaho is holding caucuses beginning at 9 p.m. ET. There are 32 delegates at stake, to be allocated on a proportional basis.
* Massachusetts is holding a primary, with polls closing at 8 p.m. ET. It is Romney's home state. There are 38 delegates at stake, to be allocated on a proportional basis.
* North Dakota is holding caucuses from 6:30 p.m. to 10 p.m. ET. There are 28 delegates at state, to be allocated on a proportional basis.
* Ohio is holding a primary, with polls closing at 7:30 p.m. ET. There are 63 delegates at stake, to be awarded on a proportional basis.
* Oklahoma is holding a primary, with polls closing at 8 p.m. ET. There are 40 delegates at stake, to be allocated on a proportional basis.
* Tennessee is holding a primary, with polls closing at 8 p.m. ET. There are 55 delegates at stake, to be awarded on a proportional basis.
* Vermont is holding a primary, with polls closing at 7 p.m. ET. There are 17 delegates at stake, to be awarded on a proportional basis.
* Virginia is holding a primary, with polls closing at 7 p.m. Only Romney and Paul qualified to be on the ballot. There are 46 delegates at stake, to be allocated on a proportional basis.
CNN' s Jim Acosta, Rachel Streitfeld, Martina Stewart and Dana Bash contributed to this report.