This year marked the 20th anniversary of the Partisan Voter Index (PVI). PVI is a measurement of how partisan a district or state is. It is calculated using results of the last two presidential elections. For example, a district with the score of D+5 means that in the 2012 and 2016 presidential elections, that district performed an average of five points more Democratic than the nation did as a whole.

With 2017 coming to a close, our friends at Quorum analyzed what the PVI of a district can reveal about how often a member of Congress works across the aisle. One of their key findings is that members in less partisan districts vote against their party at a higher rate than a member in more partisan districts.

Quorum found that members in swing districts (D+5 to R+5) vote against their party an average of 7.5% of the time.

Here's a list of the top ten Republicans and Democrats who vote against their parties most often. Many are in some of the least partisan and/or most competitive districts

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