Well, it happened simply because we were not able to discipline ourselves according to voter sentiment. We kept making that mistake- this foolishness- about you got to be this progressive or that progressive.
That phrase- "defund the police"- caused Jaime Harrison tremendously. I'm not saying it was the only problem.
Although he claimed "I don't blame progressive members," he maintained
When you ask somebody "why would you want to defund the police? They'll tell you "That's not what we mean. This is what we mean." My position is, in politics, the moment you start explaining what you mean, you are losing the argument.
Between those two remarks, Clyburn emphasized "Stop sloganeering. Sloganeering kills people. Sloganeering destroys movements. Stop sloganeering."
On NBC, Clyburn said much the same, arguing
And I also can tell you about the Senate here in South Carolina. Jaime Harrison started to plateau when defund the police showed up with a caption on TV. That stuff hurt Jimmy and that's why I spoke out against it a long time ago. I've always said that these headlines can kill a political effort.
Funny thing about that, though. In mid-October, Harrison Senate candidate Harrison had told an interviewer "There needs to be some reform to our criminal justice system," a position approximately 93 people in the country, maybe 3 or 4 in South Carolina, would disagree with. Far more significantly, he added "I oppose all of these efforts to defund the police."
Nonetheless, the unpopular slogan may have critically injured the Democratic nominee, as Clyburn contends. But it would not have had nearly the impact unless it were credible, and Harrison made it clear he stood behind law enforcement. Instead, the slogan may have have been a proxy for a movement no Democrat (and few Republicans, actually) dare question.
Among House Democrats who lost their seat on November 3, few if any supported defunding the police. Overall, few Democrats specifically endorsed the concept, let alone the slogan, of defunding the police. There was very little of "defunding the police means giving greater support to housing, health, and education." The idea was opposed or ignored.
But maybe Democrats were perceived by many voters as supporting police defunding because it of their overwhelming support of another social justice movement- black lives matter or, as it appears in writing, Black Lives Matter.
CNN exit polling found 57% of respondents have a "favorable view" of Black Lives Matter and 37% an "unfavorable view." However, that same poll found that 49% believe the "economy" is "excellent/good" ("not good/poor," 50%).
That's no typographical error. With an economy worse than at any time since the Great Depression, roughly half of Americans allegedly believe it is in good or excellent shape. Opinions as to why, or whether President Trump is wholly, partially, or not at all responsible can differ. However, the notion that the economy is buzzing along smoothly is bizarre.
Thus, it is unlikely there is an actual +17 for Black Lives Matter (if at the polls) or black lives matter (if by phone). Probably, a large portion of whites heard or read "black lives matter" and were determined not to cast a vote for the notion that black lives don't matter because the vast majority of whites do believe the lives of blacks matter- or at least that they mustn't admit otherwise.
For many voters, an attitude toward defunding the police may be a proxy for an attitude toward the summertime protests of racial inequity. The raison d'etre of Black Lives Matter is a national defunding of police- yet, voters claim they approve of it. Something, as they say, is not kosher.
It is likely that Clyburn et al. are on to something, and that prevalence of the "defund the police" slogan hurt Democrats who stayed a mile away from both the slogan and the concept. Asked a question about "black lives matter," voters nod yes; remind them of urban violence and their attitude changes- and it affects their vote.
Not all slogans are strategically damaging, of course. Nonetheless, Democrats have practiced inept messaging in recent years, and, however counter-intuitive, Black Lives Matter probably is no exception.