Romney: McCain is the "Big Dog"

In a classic quote from Mitt Romney via Politico:

Listening to Obama and Clinton discuss their national security credentials, Romney said, is akin to "listening to two chihuahuas argue about which is the biggest dog."

"When it comes to national security, John McCain is the big dog, and they are the chihuahuas," he said.

The question is: does being associated with McCain help or hurt Romney's presidential ambitions?