Almost everyone was caught by surprise when then-House Majority Leader Eric Cantor was unseated in the Republican primary by little-known professor Dave Brat, but Cantor's team actually did see Brat as a threat months before the primary. However, their response may have done a lot more harm than good. Cantor ran a spot accusing Brat of being a liberal adviser to then-Democratic Virginia Gov. (and now Sen.) Tim Kaine. The evidence they gave was pretty thin: Brat served on the Council of Economic Advisers when Kaine was trying to raise taxes.
Cantor's team thought this was a good way to disqualify Brat before he could respond. However if anything, all they did was increase Brat's name recognition. The fact that the attack was so weak may have even made Cantor look all the worse. With Cantor so unpopular among voters, his campaign could have at least attempted to raise his approval ratings, but they didn't even try. Instead, all they did was provide Brat with more name recognition.