One of the main narratives assigned to the Republican primary season is the quest to be the "anti-Romney", the candidate who gains the most support among conservatives who think Romney is too moderate (and/or too Mormon, depending on your view of the Christian Right). With Romney running for the Republican nomination non-stop for the past 5 years, voters have already made their minds up on him, and his overall support is unlikely to change much, at least until actual primary and caucus results start shaking up the race.
The polling data pretty much supports this view. Despite big shake-ups to the race (Huckabee dropping out, the rise and fall of Trump and Bachmann (and Perry?), the rise of Herman Cain), Romney's support in polls has been pretty steady in the high-teens to mid-twenties.
His support is high enough to dominate a field of candidates who split the conservative votes among themselves. The goal of all the other candidates is to consolidate the anti-Romney vote behind him-/herself, climbing above Romney's steady but beatable support. Romney wins by keeping the field closely competitive with each other, allowing his more moderate voter base to carry him to victory. He is greatly helped by additional conservatives in the race; Sarah Palin entering the race would probably deliver Romney the nomination. She would pull enough conservatives from Bachmann and Perry in Iowa that Romney would likely win there. Following that up with a win in New Hampshire, where he absolutely dominates the polls, would pretty much end the race. In the past 9 election cycles, which more or less define the modern primary era, no Republican has won the nomination without winning either Iowa or New Hampshire, and wins there would coalesce support behind him as the most electable candidate. There would be immense pressure from anti-Romney conservatives on weaker conservative candidates to get out of the race before Iowa to keep that from happening.
However, the emergence of another moderate candidate, such as NJ Gov. Chris Christie, or the ascension to viability of former Utah Gov. Jon Huntsman, would likely devastate Romney's campaign. If either is able to peel 5-10% from Romney's total, he sinks from frontrunner to also-ran.
Christie will reportedly announce his decision in the next few days. We'll know a lot more about the shape of the Republican race, and Mitt Romney's chances, when he does.