The seat is from CA. The Obama's are currently residents of DC. So "Mike" won't be eligible to take the CA seat.

Kamala is from CA however, and Newsom has promised to nominate a black woman for the position. Kamala is the obvious choice. She is an ex-senator and currently the president of the Senate. The optics would be that she would sacrifice and move there permanently again for the good of the state, as there would be nobody else they could find in time. The last vacant senate appointment took 5 months to fill. Nobody would object to appointing Kamala to fill the vacant seat.