Presumably you have him permission to search the cooler, or no? If you did, that's such a shame, a cooler in your front seat is by no means an extraordinary deviation from customary practice by the general public.

What I'm saying is, having a cooler in the front seat is not probable cause sufficient to warrant a search, plain and simple. While officers, often times ignorant regarding the scope of probable cause, abuse their powers and take advantage of citizens to facilitate illegal, unwarranted searches, a good lawyer [ ;) ] could most likely get you off, the exclusionary rule, while limited in scope, can be a defendants best friend.

Though of course, you could have told him you do not consent to any search, and, much like the officer, have a reasonable expectation of privacy. They'd definitely threaten to bring the k-9's out (call a unit, whatever), but usually, it just remains a threat and action is typically not taken, unless you have a prior record, and the officer is almost 100% certain you have drugs on your persons.

Even if they brought the dogs out, and the dogs detected the substance, you'll still be provided your day in court, where you can test the reliability of the specific drug dogs ability to detect (though not saying you'd get off) illegal substances. If the dog proves to be unreliable (happens quite often) your charges could be potentially lessen (probably not, unfortunately), but you'd in turn have a civil cause of action against the office for violation of your 4th amendment right, and potentially be awarded damages.

tl;dr: If a cop asks to search your car, don't consent to a search, ever (assuming warranted probable cause of his end doesn't exist)