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Chance the Democrat wins (21.0%)
Chance the Republican wins (79.0%)
We'll be updating our forecasts every time new data is available, every day through Nov. 6.
The Classic version of our model projects a race’s outcome by taking a weighted average of polls of a district (if available), polls of similar districts (CANTOR) and non-polling factors (fundamentals). It is then reverted toward a mean based on long-term trends in midterms and presidential approval ratings.
We've collected one poll for the California 21st. We’re adjusting poll results in three ways: Polls of registered voters or all adults are adjusted to a likely-voter basis; older polls are adjusted based on shifts in the generic congressional ballot since the poll was conducted; and polls are adjusted for house effects (the tendency for a firm’s polls to lean toward Democrats or Republicans). Polls with larger sample sizes and those conducted by higher-quality polling agencies are given more weight, as are more recent polls.
|margin||likely voter||Time-line||House effects||Adjusted margin|
A = adults
RV = Registered voters
V = voters
LV = likely voters
= partisan poll
Our district similarity scores are based on demographic, geographic and political characteristics; if two districts have a score of 100, it means they are perfectly identical. These scores inform a system we use — CANTOR, or Congressional Algorithm using Neighboring Typologies to Optimize Regression — to infer what polling would say in unpolled or lightly polled districts, given what it says in similar districts.
|Sim. score||Polling avg.|
The Classic and Deluxe versions of our model use several non-polling factors to forecast the vote share margin in each district.
|David Valadao has been elected to 3 terms. Congress has only a 20.1% approval rating, reducing the incumbency advantage.|
|CA-21 is 10.1 percentage points more Democratic-leaning than the country overall, based on how it has voted in recent presidential and state legislative elections. It voted for Clinton in 2016 and Obama in 2012.|
|Incumbent's margin in last election|
|Valadao won by 13.5 percentage points in 2016.|
|Democrats lead by an average of 8.6 percentage points in polls of the generic congressional ballot.|
|As of Oct. 17, TJ Cox had raised $1,719,000 in individual contributions (59% of all such contributions to the major-party candidates); Valadao had raised $1,176,000 (41%).|
|Incumbent's voting record in Congress|
|Valadao has voted with Republicans 79% of the time in roll-call votes in recent sessions of Congress.|
|Cox has never held elected office.|
|Neither candidate is involved in a scandal.|
|Top-two primary margin|
|Republicans won the aggregate vote by 25.6 percentage points in the June primary. However, turnout patterns can differ in the general election, so the model compares the primary result to others in California. On that basis, it treats the primary result as equivalent to a 12.1-point win for Republicans.|
The Deluxe version of our model calculates an implied margin for each race based on expert race ratings from The Cook Political Report, Inside Elections and Sabato's Crystal Ball; it then adjusts that margin toward its estimate of the national political environment.
|Cook Political Report||R+12.1||R+11.4|
|Sabato's Crystal Ball||R+6.7||R+7.1|
Nate Silver explains the methodology behind our 2018 midterms forecast. Read more …