It's no secret that Cruz is hated by McCain and lots and lots of the Washington establishment. McCain famously/infamously called Cruz (and a few other tea party types) "wacko birds" — he later apologized — and has repeatedly clashed with the Texas senator over what he believes to be the latter's tendency to grandstand. (McCain detractors will roll their eyes at the idea of him being apoplectic over other people grandstanding.)
What has been less clear is how the GOP establishment's white-hot hatred for Cruz could manifest itself in the Republican presidential primary. This is how.
By McCain giving a "you know, that's a good question" response to the question of whether Cruz is eligible to be president, he keeps the story — not a good one for the Texas senator — very much alive.
Now, the media narrative becomes that it's not just Donald Trump saying things because (a) he's Donald Trump and (b) it's in Trump's political interest to attack Cruz since he trails him in Iowa. It's the 2008 Republican presidential nominee who, oh by the way, went through a very real back and forth over his own eligibility to run back then.
McCain shivving Cruz won't drastically damage the Texas senator's strength in the race. For some, it will affirm that Cruz is the anti-establishment force they love. But, McCain's comment shows how the establishment can influence the race — by using the platform senators (and others) have to kill or boost stories.
Expect more of this. And the establishment will relish every minute of it.