Want to know the key to former Massachusetts governor Mitt Romney winding up as the Republican presidential nominee? Two words: Ron Paul.

The Texas Congressman’s strength in Iowa — there is a legitimate case to be made that he will win the Jan. 3 caucuses but more on that later — coupled with his willingness to go after frontrunning former House Speaker Newt Gingrich in often quite personal terms make him perhaps the critical x-factor in Romney’s winning calculus.

DES MOINES, IA - DECEMBER 10: Former speaker of the House Newt Gingrich (C) speaks while former Massachusetts Gov. Mitt Romney (L), and U.S. Rep. Ron Paul (R-TX), look on during the ABC News GOP Presidential Debate on the campus of Drake University on December 10, 2011 in Des Moines, Iowa. (Photo by Kevork Djansezian/Getty Images)

Winning in Iowa is — and always was — going to be a very heavy lift for Romney.

His problem in the state this time around is very similar to his problem four years ago. Socially conservative evangelicals simply don’t like or trust Romney — whether that’s because of past flip flops on issues like abortion and gay marriage or because of his Mormon faith is up for debate — and will almost certainly vote for the most viable alternative candidate.

Gingrich is clearly that candidate (although we continue to believe Texas Gov. Rick Perry has the potential to make a late run for that vote) at the moment. And, a win in Iowa would almost certainly catapult Gingrich into contention in New Hampshire, a state where Romney absolutely must win in order to preserve his path the the nomination.

Enter Paul, who may well be the only candidate who can beat Gingrich in Iowa at this point.

While Gingrich is currently leading the Iowa pack, there is a widespread expectation that he will underperform his poll numbers due to the lack of anything close to a first-class organization, and the likelihood that he will take heavy incoming from his rivals over the next three weeks.

Paul has already begun that onslaught, airing an ad in Iowa that paints Gingrich as a serial hypocrite who doesn’t really believe in the conservative principles he is espousing.

And, Paul’s distaste toward Gingrich seems to go beyond the political and into the personal. (Remember that Paul served under Gingrich when the latter was Speaker in the mid 1990s).

Two examples: 1) Asked about running such a tough ad on Gingrich, Paul responded: “I have to expose him for what he’s been doing over the years.” WHAM. 2) In Saturday night’s Iowa debate, Paul was asked again about his attacks on Gingrich and said: “Well, he’s been on different positions, you know, on so many issues. You know, single payer-- he’s taken some positions that are not conservative.” POW.

If Paul and Romney — two of the best funded candidates in the Iowa field — are both going full bore at Gingrich on television in the final three weeks of the race, it will make it far more difficult for the former House Speaker to maintain his current elevated standing in the Hawkeye State.

But it’s not just in helping drag Gingrich’s numbers down in Iowa where Paul can help Romney.

Paul is widely regarded as the best organized candidate in the state. He also has the most ardent supporters. Those two factors have convinced many seasoned Republican operatives that the Iowa race is really a two-person race not between Gingrich and Romney but between Gingrich and Paul.

A Paul victory in Iowa would be a dream come true for Romney. Why? Because Paul, like former Arkansas governor Mike Huckabee in 2008, has far less obvious appeal in the states beyond Iowa and would likely struggle to build his caucus victory into a broader national campaign.

Simply put: The less Iowa matters, the better for Team Romney. And a Paul victory there, while intriguing and a case study for political scientists for years to come, would almost certainly mean that the real race for the nomination begins a week later in New Hampshire.

The matchup to watch in Iowa then isn’t Newt vs Mitt. It’s Newt vs Ron. Or so Mitt hopes.