score:5
As SFO is your first port of entry into the United States, you will have to pick up your checked luggage after immigration and take them through US Customs. If you were continuing on with your itinerary as scheduled, you would then hand your bags back to an airline representative where they would be rechecked to Chicago.
However, there is nothing stopping you from exiting the customs area and walking right out of the airport with them instead. This is true even if your bags are tagged through to Chicago, and there's no need to mess around with asking them to short-check your bags, which is just advertising your intent to violate the conditions of your ticket. They are your bags and you are free to leave with them if you want.
Note though that if you do this, the airline will automatically cancel all of the remaining segments on your itinerary. The SFO-ORD leg will be wasted and you'll need to find another way to continue your journey. If this is the first-half of a round-trip ticket, your return trip will be cancelled. Let me say that again because it often leads to misunderstandings and heartbreak: if you miss one segment, the entire rest of your itinerary will be automatically cancelled.
As noted in Do you have to take the second leg of a domestic flight?, this typically violates the airline's contract of carriage, and they may take action against you if this is something you make a habit of. Your connection in SFO also is not guaranteed: if there are delays or cancellations, the airline is contracted to get you to Chicago, not San Francisco, and so you may be rebooked on a routing that doesn't go through SFO at all.