Brian Snyder / REUTERS

Republican presidential candidate and former Massachusetts Governor Mitt Romney and U.S. Senator Rob Portman, R-Ohio, talk after a campaign rally in Cincinnati, Ohio September 1, 2012.

WEST WINDSOR, VT -- Ohio Sen. Rob Portman came close to being Mitt Romney's most important defender, but, starting Tuesday, he is the Republican presidential nominee's biggest rival.

Portman joined the Republican presidential nominee on Tuesday to help out with preparations for this fall's debates. Portman is playing the role of President Obama during these initial practice sessions located in the remote woods of Vermont. 

"It's quiet and there are no distractions," said a senior Romney adviser of the 134-acre estate of former Massachusetts Lt. Gov. Kerry Healey. It's not in a swing state and far from any major TV markets.

Portman was among the top contenders to be chosen as Romney's running mate.  But now, instead of spending his days campaigning throughout the country, he is holed up with top Romney advisers preparing for three of the most important evenings of the presidential race. Top Romney aides Beth Myers, Stuart Stevens, Lanhee Chen, Fehrnstrom, Ed Gillespie and Peter Flaherty have all descended upon the home, as well.

It's a role Portman knows well, having played Obama during Sen. John McCain's practice presidential debates in 2008.

And while this may be familiar territory for Portman, it is relatively unexplored terrain for Romney.  Even after participating in 19 debates throughout last fall and the early part of 2012, Romney never prepared by engaging in mock debates.  Instead, advisers say, the former Massachusetts governor preferred to sit with advisers and float possible questions and answers. But, a one-on-one debate is different, and that is why Portman is in The Green Mountain State.

On Tuesday March 6, 11 states across the country -- Alaska, Georgia, Idaho, Massachusetts, North Dakota, Ohio, Oklahoma, Tennessee, Vermont, Virginia, and Wyoming -- will hold contests that will award a combined 424 delegates. That’s more than any other one day this Republican primary season. Up until now, there have been 12 contests (in some form or fashion), with Mitt Romney winning seven of them, Rick Santorum four, Newt Gingrich one, and Ron Paul zero. NBC’s current delegate count stands at Romney 119, Gingrich 30, Santorum 17, Paul 8.

The GOP presidential candidates have different strategies and strongholds in these 11 contests. Romney hopes to lock down his home state of Massachusetts, Vermont, and Virginia (where only he and Paul are on the ballot). Santorum is expecting wins in Oklahoma and Tennessee. Gingrich has focused on his home state of Georgia. And Paul has concentrated on the caucuses in Alaska, Idaho, and North Dakota. The biggest prize is Ohio, where all the candidates -- except for Paul -- have campaigned. But more than anything else, Super Tuesday is a math race: Which candidate can rack up the most delegates from these 11 states? Note that many of these contests award delegates proportionally, so a second-place (or even third place) finish can get you delegates.

Click here to read the NBC News Super Tuesday Guide, complete with analysis of the candidates’ strategies, ad spending, candidate travel, history of Super Tuesday – when and why they’ve mattered, and a complete state-by-state breakdown of the delegates at stake, rules, procedures, poll opening and closing times, and full results so far.