If it's not true, then McCain should take legal action simply for the P.R. standpoint of appearing to fight for his choice, because it will be tough to prove in court that the Enquirer did this maliciously knowing up front that it was false, which is the standard that a public figure has to meet to win a libel case. Even if they can't meet that standard, though, him fighting for his wronged running mate will be a great move politically (and morally too).
“If it is true ... “
It’s not, so you can rest easy. And there will not be any lawsuit file. McCain campaign is just making a point.