Cook Partisan Voting Index
The Cook Partisan Voting Index (abbreviated CPVI or PVI) is a measurement of how strongly a United States congressional district or state leans toward the Democratic or Republican Party, compared to the nation as a whole, based on how that district or state voted in the previous two Presidential elections.   [a]
The index is updated after each election cycle. The Cook Political Report introduced the PVI in August 1997 to better gauge the competitiveness of each district using the 1992 and 1996 presidential elections as a baseline.   The index is based on analysis by the Center for Voting and Democracy (now FairVote) for its July 1997 Monopoly Politics report. 
PVIs are calculated by comparing a congressional district's average Democratic or Republican Party share of the two-party presidential vote in the past two presidential elections to the national average share for those elections. For example, the national average for 2004 and 2008 was 51.2% Democratic to 48.8% Republican.  In Alaska's at-large congressional district, the Republican candidate won 63% and 61% of the two-party share in the 2004 and 2008 presidential elections, respectively. Comparing the average of these two district results (62%) against the average national share (48.8%), this district voted 13.2 percentage points more Republican than the country as a whole, or R+13. 
Prior to its April 2009 update, the PVI formula compared district-level results for the past two presidential elections to nationwide results for only the most recent election. Since then, local elections are compared to synchronic national elections. [ better source needed]
The Cook PVI is displayed as a letter, a plus sign, and a number. The letter (either a D for Democratic or an R for Republican) reflects the major party toward which the district (or state) leans. The number reflects the strength of that partisan preference in rounded percentage points. A district or state that "performed within half a point of the national average in either direction" is designated as "EVEN". 
The PVIs for congressional districts are calculated based on the 2012 and 2016 presidential elections.  The party representations are based on the current members of the 117th Congress. In the House, there are 241 districts more Republican than the national average, and 194 districts more Democratic than the national average.  The number of swing seats, defined as those between D+5 and R+5, is 72. 
|New Hampshire 1||R+2||Democratic|
|New Hampshire 2||D+2||Democratic|
|New Jersey 1||D+13||Democratic|
|New Jersey 2||R+1||Republican|
|New Jersey 3||R+2||Democratic|
|New Jersey 4||R+8||Republican|
|New Jersey 5||R+3||Democratic|
|New Jersey 6||D+9||Democratic|
|New Jersey 7||R+3||Democratic|
|New Jersey 8||D+27||Democratic|
|New Jersey 9||D+16||Democratic|
|New Jersey 10||D+36||Democratic|
|New Jersey 11||R+3||Democratic|
|New Jersey 12||D+16||Democratic|
|New Mexico 1||D+7||Democratic|
|New Mexico 2||R+6||Republican|
|New Mexico 3||D+8||Democratic|
|New York 1||R+5||Republican|
|New York 2||R+3||Republican|
|New York 3||D+1||Democratic|
|New York 4||D+4||Democratic|
|New York 5||D+37||Democratic|
|New York 6||D+16||Democratic|
|New York 7||D+38||Democratic|
|New York 8||D+36||Democratic|
|New York 9||D+34||Democratic|
|New York 10||D+26||Democratic|
|New York 11||R+3||Republican|
|New York 12||D+31||Democratic|
|New York 13||D+43||Democratic|
|New York 14||D+29||Democratic|
|New York 15||D+44||Democratic|
|New York 16||D+24||Democratic|
|New York 17||D+7||Democratic|
|New York 18||R+1||Democratic|
|New York 19||R+2||Democratic|
|New York 20||D+7||Democratic|
|New York 21||R+4||Republican|
|New York 22||R+6||Uncalled|
|New York 23||R+6||Republican|
|New York 24||D+3||Republican|
|New York 25||D+8||Democratic|
|New York 26||D+11||Democratic|
|New York 27||R+11||Republican|
|North Carolina 1||D+17||Democratic|
|North Carolina 2||D+9||Democratic|
|North Carolina 3||R+12||Republican|
|North Carolina 4||D+17||Democratic|
|North Carolina 5||R+10||Republican|
|North Carolina 6||D+9||Democratic|
|North Carolina 7||R+9||Republican|
|North Carolina 8||R+8||Republican|
|North Carolina 9||R+8||Republican|
|North Carolina 10||R+12||Republican|
|North Carolina 11||R+14||Republican|
|North Carolina 12||D+18||Democratic|
|North Carolina 13||R+6||Republican|
|North Dakota at-large||R+16||Republican|
|Rhode Island 1||D+14||Democratic|
|Rhode Island 2||D+6||Democratic|
|South Carolina 1||R+10||Republican|
|South Carolina 2||R+12||Republican|
|South Carolina 3||R+19||Republican|
|South Carolina 4||R+15||Republican|
|South Carolina 5||R+9||Republican|
|South Carolina 6||D+19||Democratic|
|South Carolina 7||R+9||Republican|
|South Dakota at-large||R+14||Republican|
|West Virginia 1||R+19||Republican|
|West Virginia 2||R+17||Republican|
|West Virginia 3||R+23||Republican|
The District of Columbia's at-large congressional district, represented by a non-voting delegate, is ranked by Cook PVI, as it participates in presidential elections. Its PVI is D+43.  Territorial districts are not ranked on the Cook PVI, as they do not participate in presidential elections.
|New Jersey||D+7||Democratic||Democratic||10D, 2R|
|New Mexico||D+3||Democratic||Democratic||2D, 1R|
|New York||D+12||Democratic||Democratic||19D, 7R, 1 Vacant|
|North Carolina||R+3||Democratic||Republican||8R, 5D|
|South Carolina||R+8||Republican||Republican||6R, 1D|
*Includes an independent senator who caucuses with the Democrats.
The most Democratic congressional district in the country is New York's 15th, located in the Bronx, with a PVI of D+44.  The most Republican district is Texas's 13th, based in the Texas Panhandle, at R+33.  In terms of states as a whole, Wyoming is the most Republican at R+25, and Hawaii is the most Democratic at D+18. 
In the Senate, the most Republican-leaning state to have a Democratic senator is West Virginia (R+19 PVI), represented by Joe Manchin. The least Democratic-leaning state to have two Democratic senators are Arizona (R+5 PVI), represented by Mark Kelly and Kyrsten Sinema, and Georgia (R+5 PVI), represented by Jon Ossoff and Raphael Warnock. The most Democratic-leaning state to have a Republican senator is Maine (D+3 PVI), with Susan Collins. The least Republican-leaning state to have two Republican senators is Florida (R+2 PVI), represented by Marco Rubio and Rick Scott.
Four states with a Republican-leaning PVI (Kansas, Kentucky, Louisiana, and North Carolina) have Democratic governors, while three Democratic-leaning states (Maryland, Massachusetts, and Vermont) have Republican governors. These governors are all seen as centrists who attracted voters from the opposite party. The most Republican-leaning state with a Democratic governor is Kentucky (PVI R+15), with Andy Beshear, and the most Democratic-leaning state to have a Republican governor is Vermont (PVI D+15), with Phil Scott. Two of the three states with even PVI ratings, Pennsylvania and Wisconsin, have Democratic governors ( Tom Wolf and Tony Evers, respectively).
In the House, the most Democratic-leaning congressional district represented by a Republican is Florida’s 26th; with a PVI of D+6, the district is represented by Carlos Gimenez, who is one of only six Republicans to represent a Democratic-leaning House district. The most Republican-leaning congressional district represented by a Democrat is Georgia’s 7th with a PVI of R+9, it is represented by Carolyn Bourdeaux. Following the 2020 elections, there were 26 Republican-leaning House districts represented by Democrats. The only Republican leaning state to have more Democratic House members than Republican House members is Arizona (PVI R+5) with 5 Democrats and 4 Republicans. The most Democratic leaning state to have more Republican House members than Democratic House members is Wisconsin (PVI EVEN), with 5 Republicans and 3 Democrats. The most Democratic district relative to its state is Tennessee's 9th, being D+28 in an R+14 state (a 42-point difference). The most Republican relative to its state is Illinois's 15th, being R+21 in a D+7 state (a 28-point difference). The most Republican states to have a Democratic Representative are Alabama, Kentucky, and Tennessee; while the most Democratic states to have a Republican Representative are California, Maryland, and New York. Of the 428 Congressional districts that are in states with more than one district, 104 lean to one party while their state leans to the other.
- Political party strength in U.S. states
- Psephology, the statistical analysis of elections
- Two-party-preferred vote
- “Essentially, the Cook PVI uses national and state results from the last two presidential elections to calculate how each state leans relative to the country as a whole. For example, if a Democratic presidential candidate won the popular vote nationally by five points, he or she might win a D+2 state (which leans two points toward Democrats) by seven points. And if a Republican won the popular vote by three points, he or she might win a D+2 state by one point.” — D. Byler (2016) 
- Cillizza, Chris (March 14, 2018).
"The differences between real grassroots and 'Astroturf' matter". CNN. Retrieved November 29, 2020.
Which brings me to the Cook Political Report's Partisan Voting Index or PVI. The goal of the PVI is to compare every congressional district to every other congressional district based on how it has performed in each of the last two presidential elections.
- Benen, Steve (February 7, 2017).
"There are 119 Republican House members who should be VERY nervous today". MSNBC. Retrieved November 29, 2020.
To get a sense of a congressional district's political leanings, there's a helpful metric called the Partisan Voter Index, or PVI, created 20 years ago by the Cook Political Report.
- Byler, David (October 7, 2016). "Trump trades traditional Republicans for swing-state voters". RealClearPolitics. Retrieved November 29, 2020.
- Wasserman, David (October 11, 2012). "Introducing the 2012 Cook Political Report Partisan Voter Index". The Cook Political Report. Archived from the original on May 4, 2013. Retrieved November 17, 2012.
- Wasserman, David; Flinn, Ally (April 7, 2017). "Introducing the 2017 Cook Political Report Partisan Voter Index". The Cook Political Report. Retrieved November 29, 2020.
- "Monopoly Politics". fairvote.org. Center for Voting and Democracy. July 1997. Retrieved November 17, 2012.
- "Partisan Voting Index Districts of the 113th Congress: 2004 & 2008" (PDF). The Cook Political Report. 2012. Archived from the original (PDF) on April 29, 2013. Retrieved November 29, 2020.
- Nir, David (February 6, 2009). "Swing State Project:: A Look at the Cook Political Report's Partisan Vote Index (PVI)". Swing State Project. Archived from the original on February 24, 2009. Retrieved August 5, 2017.
- "PVI Map and District List". The Cook Political Report. Retrieved January 8, 2021.
- "State PVI Map and List". The Cook Political Report. Cook Political Report. Retrieved November 29, 2020.