I actually blame Democrats for this, and the Circular Firing Squad has been taking out good candidates for decades now including Edward Muskie, Gary Hart, Howard Dean, and Amy Klobuchar. In its place, we have gotten George McGovern, Walter Mondale, John Kerry, and Joe Biden, each of whom was significantly worse, and much more a pushover, than the good candidates.

(note I was not of voting age in 1984 so I'm not sure if Hart's scandals were truly red flags. But Hart's replacement was hardly an improvement)