Welcome to the biggest day of the 2012 Republican presidential primary!

Ten states across the country are set to have their say in the GOP nomination contest with more than 400 delegates at stake — more than in the entire first two months of the race combined.

With so much happening, it’s hard to know where to look and to figure out what really matters. That’s where we come in! Below are five of the biggest storylines we are following tonight. Got one that we didn’t mention? Add it in the comments section.

1. Define “winning” : It’s an impossibility that any of the four remaining Republican candidates will sweep all ten states voting today. Given that, defining victory is tough.

Former House Speaker Newt Gingrich will almost certainly declare Super Tuesday a success despite the fact that he is likely to only win his home state of Georgia. Ditto Texas Rep. Ron Paul who should do well in low turnout caucuses in North Dakota, Alaska and Idaho.

But the real race for a Super Tuesday winner comes down to former Massachusetts governor Mitt Romney and former Pennsylvania senator Rick Santorum who, between them, will likely win the lion’s share of states.

Romney will win Vermont and Massachusetts. Santorum is likely to take Oklahoma and Tennessee. That makes Ohio the jump ball. Polling shows the Buckeye State as a dead heat although Romney clearly is the momentum candidate.

Win Ohio and you have won the night. It’s that simple.

2. Can Romney end it?: From a delegate point of view, Romney is nowhere near clinching the nomination. (Check out our video explaining all of the delegate math.)

But, there is a path toward him closing out the nomination — for all intents and purposes — tonight. How? Romney needs to be able to claim a sort of national victory, winning somewhere in every region of the country.

The Northeast is locked up as Romney will cruise in his home-ish state of Massachusetts and Vermont. He’s likely to get a win (if not two) out of the Plains/West with the North Dakota and Idaho caucuses. Ohio is Romney’s chance in the Midwest/ Rust Belt.

That leaves the South. Gingrich is going to win Georgia. Santorum looks strong in Oklahoma and it’s somewhat debateable whether that counts as the South anyway. Tennessee is clearly Romney’s best chance to win in the South even though polling suggests that Santorum has a narrow edge.

Restore Our Future, the Romney-aligned super PAC, spent upwards of $1 million on television ads in the Volunteer State but it remains to be seen whether Romney’s momentum can overcome the early vote which Santorum almost certainly won.

If Romney wins — for the sake of argument — Ohio, Tennessee, North Dakota, Idaho, Vermont and Massachusetts — he can make a compelling case to the Republican establishment, which has been loathe to get off the sidelines thus far in the race, that he is the only national candidate left in the field.

Whether they go along with that argument is a different question.

3. Ron Paul, winner?: The little secret behind the movement that is Ron Paul’s presidential campaign is that he has yet to win a state — or even come all that close — in either his 2008 or 2012 bids.

Super Tuesday represents his best chance — by far — to break that streak. In 2008, Paul won 24 percent of the vote — his best showing in any state — although he still lost badly to Arizona Sen. John McCain.

Idaho is, again, an opportunity for Paul. But, it’s in Alaska where Paul may finally breakthrough. Paul stopped in the Last Frontier over the weekend — the only one of the four candidates to make the trek to the state.

There are two other factors that suggest potential Paul-mentum in Alaska. First is the expected low turnout. Just 11,000 people voted in the 2008 Alaska caucuses, which Romney won with 44 percent of the vote. Paul has shown an ability to do well in low turnout elections thanks to his in­cred­ibly loyal base of supporters.

Second is that Paul’s message of libertarianism would seem to be a perfect fit for the mindset of most Alaska residents. If Paul’s “get the government out of our lives” schtick can work anywhere, it’s here.

4. Gingrich coming back, again: Imagine this scenario: the former House Speaker wins convincingly in Georgia tonight. Then a week from tonight he sweeps the Alabama and Mississippi primaries. (Aside from those two southern states, Hawaii’s caucus is the only vote set for March 13.) Isn’t it possible we are talking about Gingrich as the conservative alternative to Romney (again) (again)?

Gingrich certainly believes so and continues to give every indication he’s in the race for the long-ish haul. Heck, he’s even getting Secret Service protection starting tonight.

The problem for Gingrich is that he’s Gingrich. Polling suggests that even Republican voters are far from sold on him and he has never shown any aptitude in this race to raise the money or do the organizational blocking and tackling he would need to capi­tal­ize on the momentum he could well build over the next seven days.

And, even if he does sweep Georgia, Alabama and Mississippi, the onus will be on Gingrich to prove that he is something more than just a regional candidate — given that his only victory in the race to date was in the Jan. 21 South Carolina primary.

5. Watch conservatives: From the get-go, Romnney has struggled to convince those identifying themselves as “very conservative” that he is their guy.

In Michigan’s primary last week, Romney lost “very conservative” voters 50 percent to 36 percent to Santorum despite winning the state. In Florida’s Jan. 31 primary, Gingrich took 41 percent to 33 percent for Romney among very conservative voters despite the fact that Romney won the Sunshine State.

With 10 states on the ballot, we will get our broadest look yet at whether Romney has many any inroads into solving that problem. While he’s never going to be beloved among the conservative wing of the party, Romney needs to be at least be-liked (is that a word) in order to put down the “unrest on the right” narrative.

Remember that conservatives may not love Romney but they dislike President Obama far more. If Romney’s numbers do bump up today from his past showing among very conservative voters, it’s likely due to the fact that electability — always Romney’s strongest characteristic 00 has come to the fore.

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