No way. You just gave permission to search, and the OP wants to find a polite way to decline -- you accomplished the opposite of what the OP wanted. Asking permission to search is an officer's job; there is absolutely nothing intimidating about asking for his/her supervisor to be there.
There are several levels of intrusion the police can make into someone's private lives. Each level of intrusion requires a different level of proof. The lowest level is called "consent" and carries NO BURDEN OF PROOF WHATSOEVER. It means the person being addressed is free to leave at any time and is present of their own volition. The same holds true for a consensual search -- the person is free to leave and does not have to allow the search to take place; they are doing it of their own volition.
The second level of intrusion is "investigatory detention," and the corresponding burden of proof is "reasonable articulable suspicion." At this point, a suspect is NOT under arrest, but there is enough proof that they are NOT free to leave, either. There are thousands of court rulings on the exact definitions.
The third level is arrest, and the corresponding burden of proof is "probable cause." A search of a home requires this level of proof, and almost always (very few exceptions) requires a warrant signed by a judge. However, many searches of a vehicle can be conducted without a warrant, but require this level of proof as well. If this level has been reached, you cannot deny the officer the search -- he/she is going to search whether you like it or not, and interfering with them will land you in jail. I have had more than one driver (or even passenger) loudly scream about how they did not consent to me searching their vehicle, to which my response was, "I've noted your objection. Now please get out of the way before I arrest you."
The fourth level is conviction, and the corresponding burden of proof is "beyond a reasonable doubt."
But this question really rooted in the first level of intrusion -- consent. In these cases, the driver DOES NOT have to allow the search. The Supreme Court has multiple rulings on this one. For example, the driver MUST BE FREE TO LEAVE before the officer asks for consent. In other words, if the officer is holding the driver's license, registration, and proof of insurance, they are not free to leave. If they are detained in the back of a patrol cruiser, they are not free to leave. You get the point.
The average reader is asking why on earth someone carrying an illegal item would give consent, and I'm here to tell you, they absolutely do. I don't know why, but I know they do. And I followed ALL the rules -- driver free to leave, use the word "search" instead of a euphemism like "look around," etc. I got more dope off consent searches than I can even recall.
So back to the original question: How to decline. I'd advise you just to do it politely, such as, "Officer, I'd rather you didn't." He/she may ask a few follow up questions -- let's face it, any good cop is suspicious and may assume you are hiding something by declining, but we all know that isn't true. Some people just don't want another person nosing around in their stuff, whether they have something to hide or not. The rookies may even ask, "Are you hiding something?" I would simply respond, "No, I just don't want to take the time out of my day to have another person snooping around in my stuff."
If the officer demands the search, or informs you they are searching your vehicle with or without your consent, one of two things is happening: 1) They have a legal reason to, which you can fight about in court later. As I said before, engaging with them on this is a good way to get arrested. Voice your objection but do not interfere, and then file a complaint with Internal Affairs and / or consult an attorney about suing them. 2) The cop is a dumba$$ and doesn't know how to play the gam,e correctly. Unfortunately, I worked with several of these guys; they do exist. So in the case of either #1 or #2, my advice from #1 applies.