Perry's campaign strategy would consist of winning Iowa and South Carolina; he would leave Mitt Romney to win New Hampshire uncontested. He would hope to win the support of the Tea Party and appeal to the middle by casting himself as a conservative who has substance (with the implication being that Michele Bachmann and Herman Cain lack substance). Perry would also make a point of criticizing President Obama and the Democrats in tough, blunt language, and would emphasize his record as an economic conservative in Texas.
From that sketch, it's possible to derive the basics of how a Perry candidacy would affect the race. First, since Perry would be pursuing Tea Party support, it would hurt Tea Party candidates. Bachmann and Cain would be in Perry's crosshairs. Would they really lose support to Perry? Yes, quite a bit of support. History tells us that voters prefer governors as president; more importantly, they never support business candidates or House Reps. Cain and Bachmann would lose a lot of Tea Party voters who want someone with more credible experience. They'll also be looking for someone who has won a state-wide race. Also note that the Tea Party is already a bit split, and adding Perry into the mix would split it further. It would be unsurprising if only one of either Cain or Bachmann survived a few months after Perry's entry.
What about Tim Pawlenty, the former governor of Minnesota? He's essentially trying to do the same thing Perry would, arguing he's a conservative (Tea Partier) who has the credibility to get elected. Pawlenty has a better electability argument than Perry, since Perry's similarity to George W. Bush and talk of secession will leave many voters cold. But at the same time, there's a reason why the field has a vacuum in it: Pawlenty isn't perceived as tough enough. He's a consensus candidate, but he lacks charisma and the "toughness" that people temporarily appreciated about Donald Trump. Thus, Pawlenty would be weakened by Perry as well.
Which brings us to Chris Christie. Christie has the ability to unite the Tea Party and the Republican establishment, and would be better at appealing to the establishment than Perry. But Christie would also be seeking to fill the same "strong leader" and "tough talk" attributes that Perry exudes and Pawlenty lacks. Because of concerns about electability, Perry would not be able to fill the vacuum the same way Christie can. But Perry would at least reduce the size of the vacuum. Any candidate who can do that would harm Christie's chances.
As we saw with the Iowa Caucus winner scenarios, Romney is most afraid of a highly electable candidate emerging from Iowa. Pawlenty and Christie are both threatening to Romney. Perry harms Pawlenty and Christie while not harming Romney. Therefore, Perry's candidacy would benefit Romney. Perry might be able to defeat Romney down the road, but Romney could at least make an electability argument against Perry that he can't bring against Pawlenty or Christie.
On the other hand, Perry's candidacy would harm the chances of Bachmann and Cain, either of whom Romney would like to see win Iowa. But since Bachmann and Cain are less likely to win Iowa,
it's a net benefit for Romney. He would gladly watch Bachmann and Cain burn if it means weakening Christie or Pawlenty.
