Cruz was trying to politely refer to Trump’s henchman Roger Stone who is widely known as a political “r@tf*cker” (the guy who does the dirty stuff behind the scenes).
Stone’s fingerprints were all over the National Enquirer Cruz adultery story.
It was still a weird, creepy thing to say. If Trump had spoken of copulating with rats no one would be trying to explain it. They would just say it disqualified him from being president, since you can’t have the leader of the free world riffing on which rats he would or wouldn’t copulate with.