Something doesn’t add up.
Good question. The traditional answer is that most Republicans identified as conservatives and most Democrats did not identify as liberals. There was some truth in that once, but not quite as much as people assumed:
And it's even less true now than it was earlier: conservative Democrats are a thing of the past and most Democrats identify as liberal.
It may have something to do with people who call themselves independents but really vote Democrat. Or it could be that all the little differences add up to a significant advantage for the Democrats.
But maybe the assumption is wrong. Democrats pick up votes in states like Virginia, New Hampshire, and Nevada, but lose them in states like Pennsylvania, Michigan, and Minnesota.