My understanding of US law (which could be wrong) is that they need probable cause everywhere in the US if it's going to extend the duration of the stop.
You have to explicitly ask to leave, and they'll ask you to get out of the car anyway as soon as you say you don't consent to a search. Depending on how long it takes the dogs to arrive you could argue about it in court, but good luck.
AFAIK this is only true in a very limited set of local jurisdictions.
The use of drug dogs to manufacture probable cause is pretty well documented.