The press, by habit, likes to fixate on the American Conservative Union and its scorecard. Its scorecard, which is produced in ways that always seem to help the party leadership, had Eric Cantor with a 95% rating. The press sees that and thinks conservatives must have lost their minds to oust Eric Cantor. They're eating their own.
The American Conservative Union has long been a mouthpiece of the Republican Establishment and in the past few years has basically been K-Street's conservatives. Their scorecard reflects the Republican-ness of a member of congress far more than the conservativeness of a member of congress. Just consider that Mitch McConnell was considered more conservative in 2012 than either Jim DeMint or Tom Coburn.
In contrast to the American Conservative Union, Heritage Action for America takes a more comprehensive approach to its scorecard, it does not try to help Republican leadership look good, and is a better barometer of a congressman's conservativeness. The ACU had Eric Cantor at a 95%. Heritage Action for America has him at 53%.
It is much more accurate. Members of the media, assuming they can be taught new tricks, might want to go there from now on.