Karine Jean Pierre on Morning Joe November 4 uttered this one.  She was saying, “Donald Trump is not expanding his base…but Democrats can’t just sit on their laurels.”  It is a mashup of “sit on their hands” (refrain from acting) and “rest on your laurels” (rely on your past achievements).  Both expression refer to inaction rather than action, and sitting and resting are both sedentary actions, hence the mixup.  A big thanks to Frank King for hearing this one and passing it on.