The from what I understand the numbers usually suggest that Democrats typically receive a larger number of votes but Republicans win more constituencies, so for Democrats to win it has to be an overwhelming majority to make up for their weakness in lower population density constituencies.
Seven of the past eight popular votes were won by the Democrats, but Republicans won three of those elections.
https://www.statista.com/statistics/1035521/popular-votes-republican-democratic-parties-since-1828/