| District ID | 21st District |
| Representative | Jim Costa |
| Party Affiliation | Democrat |
| Partisan Voting Index (PVI) | D+5 |
| 93221 |
| 93223 |
| 93227 |
| 93235 |
| 93244 |
| 93271 |
| 93277 |
| 93286 |
| 93291 |
| 93292 |
| 93615 |
| 93616 |
| 93618 |
| 93625 |
| 93631 |
| 93636 |
| 93646 |
| 93647 |
| 93648 |
| 93654 |
| 93657 |
| 93662 |
| 93666 |
| 93670 |
| 93673 |
| 93701 |
| 93702 |
| 93703 |
| 93704 |
| 93705 |
| 93706 |
| 93711 |
| 93721 |
| 93722 |
| 93723 |
| 93725 |
| 93726 |
| 93727 |
| 93728 |
| 93741 |
| 93764 |