The Rhodes Cook Letter is a print publication that is published every two months. Every issue has analysis of U.S. elections. The editor is Rhodes Cook, who also authors America Votes, the books that appear every two years and contain accurate and complete election returns for president, congress, and gubernatorial elections. The books include both primaries and general elections.
The February 2014 Rhodes Cook Letter has a survey of congressional incumbents who have been defeated for re-election in the years 2000 through 2012. It finds 209 incumbents were defeated during those years. Of those 209, 167 were defeated in the general election and only 42 (20%) were defeated in a primary.
The Independent Voters Network (based in California) and Independent Voting (based in New York) both engage in activism against closed primaries. Both organizations frequently say that primary elections are far more important than general elections, and sometimes they even allege that 95% of all elections are decided in the primaries, not the general elections. However, neither organization, to date, has presented any statistical evidence for those claims.