To his credit, Kilgore understands his position in the correct terms: his target is an "ancient lefty habit," and his solution is decidedly "progressive". Say what you will about this, it's at least more coherent than modern liberal attempts to co-opt the left and claim it as their own: Kilgore knows that he's arguing against (something like) the historical left position, and he names it as such.
Still, when we tease out his substantive critique from the rhetoric, it doesn't seem particularly strong. Kilgore's argument rests on suggesting that 1) Obama's "Bittergate" controversy and 2) Sander's focus on economic issues were both politically damaging, but there's little evidence for either claim.
Bittergate - the controversy surrounding Obama's 2008 comment that working-class voters who lose jobs "get bitter, they cling to guns or religion or antipathy to people who aren't like them" - is a particularly odd crutch to lean on. Kilgore himself admits that this supposed gaffe was "big but ultimately not decisive", which would seem to call into question any assumption that the left's economic focus is a losing strategy. But was the incident even "big" in any meaningful? The media thought so, but the polls are ambivalent at best:
Remarkably, Obama's lead nearly doubled after news of his comment broke on April 12, jumping from 5.4 to 10.4 points over Clinton. They didn't tighten up again until after multiple major events, including John McCain's win of the Republican nomination, the last Democratic debate, and Clinton's win of the Pennsylvania primary.
The alternative reading of the Bittergate episode is that this quote - like Obama's "pen and phone" quote, his "spread the wealth around" quote, and his "you didn't build that" quote - was just another media spectacle that no one outside of the beltway ever cared about. Pundits looking for a horse race may get excited over such things, and right-wing partisans looking for something to complain about may latch onto them, but this sort of rhetorical maneuvering has no sure relationship to public opinion.
Kilgore compares Bittergate with various economic-focused statements made by Bernie Sanders, but not much evidence of political damage there, either. For instance, though he claims that Sanders got "into hot water" during his supposed conflict with Black Lives Matter earlier this year, Matt Bruenig observed that most of the evidence for this was purely anecdotal. Kilgore also argues that the economic-focused message is hurting Sanders "with immigration and terrorism", but the latest YouGov / Economist poll suggests a different story:
Family Income $50,000 or Less
Whatever Sanders' economic message is doing, it doesn't seem to be having a significant negative impact on working class perceptions of his ability to handle immigration or terrorism: on both issues, working class voters are just as uneasy with Clinton as they are with him. Clinton does have higher confidence scores - but the difference can be significantly accounted for by the greater number of voters who are simply "unsure" about Sanders.
Notably, a similar dynamic appeared when Bruenig examined Sanders' numbers among black adults: they lagged behind Clinton's, but largely part because 67% of black voters were simply unfamiliar with him. This is the trend that emerges time and time again whenever you listen to what the voters are actually saying: voters are unsure about Sanders' message, so they haven't formed an opinion about it. Liberals may see these numbers as an opportunity to make ridiculous accusations of "economic reductionism" against the left, but they would do well not to confuse popular uncertainty about left politics with opposition to it.