It may be pandering, but the Obama campaigns proved it works. Democrats are updating the existing database with voter profiles based on how they voted in local elections, which magazines they read, their bank balances and the health topics they search online. Republicans using the #NeverTrump hashtag should expect to be targeted by the Clinton campaign with custom messages on the virtues of voting Libertarian or staying home. Other messages will highlight Mr. Trump’s negatives while minimizing Mrs. Clinton’s.

Reports say her hundreds of data scientists are working especially closely with Facebook to leverage each voter’s social network. The campaign is testing which messages resonate best with each prospective voter. It will also use Facebook to remind Mrs. Clinton’s supporters to urge like-minded friends and relatives to go to the polls.

A Pew Research Center study last week uncovered a new way the Clinton campaign is using Facebook to gather more information about voters. It found that 80% of the links on the campaign’s Facebook account link to pages on the campaign’s website, where more user data can be captured. In contrast, 78% of the links on the Trump Facebook account take users to news articles on media sites, where his campaign has no ability to track users or mine their data.

Campaign strategists say big data is especially valuable in close presidential races, in which some votes matter more than others. In 2012 Mr. Obama’s data scientists ran more than 60,000 election simulations every night to figure out in which states to allocate resources, and which counties in those states were the most promising sources of votes.