I can understand their preference, but "called of God?" That's theologically dubious at best, for two reasons.
Let's consider their context of "calling": This nation is under the spell of evil liberals and liberalism, so we need to find a champion who will defeat [choose the target of your choice] and restore our nation to greatness -- you know, "the way it used to be."
We've seen that one before -- Palestine in the first century. Recall that the Jewish people of that day were living under the rule of Rome, which they resented deeply and wanted overthrown and understood well the prophecy of a Messiah, who they believed would be the person who would kick the Romans out.
But in their desire for political freedom they missed what Jesus, who of course was the Messiah, came to do -- change the entire calculus in showing the world Who God really was. They ended up rejecting Him because He turned out not to be the political leader they wanted.
Two, "calling" in this context is connected to service and sacrifice, not as a will to power. It's perfectly appropriate to "call" a pastor or other spiritual leader to a place of authority; however, in such cases he or she has been specifically raised up to give of him/herself, not simply to be "in charge" and push people around. (See John Eldredge's description of the "King" in the book "Fathered by God.")
Those Christians who want Perry to be president have missed that point. You see, they have always sought someone who will fight the battle against what they consider demonic forces (read: Those who disagree with them politically) so that they can go about their lives and not have to engage in spiritual warfare themselves -- something that God will never allow. To wit, they want the "blessings" of God but not really to know Him personally and permit Him to change who they are, making Him in their image in the process.
They have also forgotten that God Himself raises up and takes down leaders for His purposes; I'm convinced that Barack Obama and, before him, Bill Clinton became president at least in part to demonstrate to politically conservative Christians that He is, and they are not, running the show. Sadly, some still don't understand that concept -- just before the general election of 2008 I received an emergency e-mail asking folks to pray that, by some miracle, Obama would be defeated. (I responded by saying that it was inappropriate.)
This is why "anointing" a presidential candidate -- keep in mind that the term "Messiah" means "anointed one" -- is flatly dangerous. And if Perry does decide to run, he needs to understand that he would lead all the people of this great country, not simply those who agree with his politics. Sadly, I don't think he does based on his speeches and record, which is why the "calling" is bogus.