Matt is right about how hesitant liberals are to admit this. They shouldn't be. Yes, there is strong evidence that ID politics/political correctness cost the Dems some votes (check his @NiskanenCenter piece), but that doesn't mean what so many centrist liberals think it does.
First, we don't know if it's costing them more votes than it's gaining. Jettisoning ID politics may help Dems win back some white suburbanites, but what if it depresses turnout in the blue cities driving the party's red state gains?
Anyway, there's a much bigger issue at play.
Anyway, there's a much bigger issue at play.
Look at all these partisan swings. On economics, the environment, higher ed, and the FBI, something insane happened in 2016/2017. Dems shoot off in one direction, GOP in the other. Partisanship rules everything around me, amirite? 
Everything, that is, except perceptions of racial discrimination. Check these numbers from Pew, which show a sharp increase in the number of Democrats acknowledging the intractability of racism, but with NO corresponding decrease among Republicans. If anything, it's the opposite.
Or check this GSS data, which shows a growing number of white liberals acknowledging discrimination, AND ALSO no erosion among conservatives. In other words, partisan symmetry, which holds for things like higher ed and the FBI, *does not hold* for racial discrimination.
There are a lot of ways of thinking about this data, but here's one interpretation: The SJWs are winning. Identity politics is winning. Yes, it may cost Dems some votes on the margins, and that matters. But look what we're getting in return. It's a bargain at twice the price.
