Every California resident lives in multiple jurisdictions simultaneously—a county, possibly a city, state legislative districts, congressional districts, school districts, and more. These boundaries don't align. They overlap, split, and intersect in ways that shape political relationships, coalition opportunities, and ballot dynamics.
Understanding jurisdictional overlap answers questions that matter for strategy: Which legislators represent the same voters as this city council? Which school board races appear on the same ballot as this Assembly race? How is a county's population distributed across congressional districts?
Why Overlap Matters Politically
Shared constituencies create relationships. A State Senator whose district overlaps significantly with three Assembly districts has natural relationships—and sometimes rivalries—with those Assembly members. They share voters, even if their districts don't match perfectly.
Ballot co-appearance affects turnout and coattails. Races that appear on the same ballot influence each other. A high-profile congressional race can drive turnout that affects a local school board race, but only in the parts of the school district that overlap with that congressional district.
Endorsement value depends on overlap. When a city council member endorses a state legislative candidate, the value of that endorsement depends on how much of the legislative district falls within the council member's sphere of influence.
Campaign coordination requires knowing shared voter pools. Two campaigns targeting the same voters—even for different offices—can coordinate (or conflict). Overlap analysis identifies where those shared pools exist.
Finding Overlapping Districts
On any jurisdiction page, click the Overlapping Districts tab.
The page organizes overlaps by jurisdiction type (Counties, Cities, State Assembly, State Senate, Congressional, School Districts, etc.). Each section is expandable, and every overlapping jurisdiction links directly to its own page.
Reading Overlap Percentages
Overlap data always shows two percentages:
- "X% of this jurisdiction" tells you how much of the place you're viewing falls within the overlapping jurisdiction
- "Y% of the other jurisdiction" tells you how significant your jurisdiction is within that broader context
Both matter, and they tell different stories.
Example: You're viewing City Council District 6 and see:
78th Assembly District: 60% of this district, 18% of the AD
This means:
- 60% of District 6 residents also live in AD-78—AD-78 is the dominant Assembly district for this council district
- But District 6 represents only 18% of AD-78's total population—the council district is a small piece of the Assembly district
The 60% figure matters for the council member. The 18% figure matters for the Assembly member.
Strategic Applications
Finding legislative allies for local officials. A school superintendent wants to know which state legislators represent their district's population. The overlap page shows which legislators have significant population in that school district—and the percentages reveal who has the most at stake.
Scoping a legislative district's local landscape. An Assembly candidate wants to understand the local political structure within their district. The overlap page shows which cities, council districts, and school boards are wholly or partially within the Assembly district—and which have enough overlap to matter.
Assessing a ladder race. A city council member considering a run for State Assembly can see what percentage of the target Assembly district their current council district represents. If they represent 25% of the AD's population, they start with a meaningful base; if they represent 5%, they're largely unknown to the electorate they'd be asking to vote for them.
Mapping county-level legislative distribution. A county party chair wants to know how the county's population splits across Congressional districts. The overlap page shows all Congressional districts that touch the county, with percentages indicating how the population is divided.
Post-redistricting reorientation. After new lines take effect, overlap analysis shows how new districts relate to existing structures. A new Congressional district might combine parts of three old ones while also splitting cities in new ways. The overlap data makes these relationships concrete.
Important Limitations
This is population overlap, not voter overlap. The percentages are based on Census population data, not registered voters or likely voters. Actual voter overlap may differ due to registration rates, turnout patterns, or citizenship status.
Overlap doesn't guarantee ballot co-appearance. Just because two jurisdictions share population doesn't mean their races appear on the same ballot. School board elections may be off-cycle, terms are staggered, and not every overlapping race runs in the same election.
Small overlaps may be noise. A 3% overlap often results from boundary irregularities or Census geography precision limits. Don't over-interpret tiny overlaps. The platform filters out overlaps below 2%.
This reflects current boundaries only. Redistricting changes overlap patterns. Historical overlap (before the most recent redistricting) is not shown.
Common Mistakes
Confusing the two percentage directions. The most common error is misreading "60% of this jurisdiction" as "60% of the other jurisdiction." Always note which percentage you're reading and what it describes.
Assuming political alignment from overlap. Knowing that two jurisdictions share population says nothing about how those shared voters behave politically. Overlap is geographic, not ideological.
Treating small overlaps as actionable. A 5% overlap means 95% of the jurisdiction is unaffected by whatever dynamics exist in that other district. For most strategic purposes, small overlaps are informational footnotes, not action items.
Expecting real-time boundary updates. Overlap data is calculated from Census geography and updated periodically. Recent annexations or boundary changes may not be immediately reflected.
Jurisdictional overlap is the hidden structure beneath California politics. The boundaries that appear on maps are simpler than the reality of shared constituencies, staggered elections, and interconnected political relationships. Overlap analysis makes that complexity visible—and usable.