Donald J. Trump and his supporters hope to overcome sweeping demographic shifts by rallying the support of “missing white voters” — disaffected, conservative populists who sat out the 2012 presidential election.

Millions of these voters could be activated, the theory goes — enough to overcome the demographic changes that favor Hillary Clinton.

Mr. Trump may yet win this election. But if he does, it probably won’t be because of a huge influx of Republican-leaning “missing” voters.

There has been no surge in registration among white voters since 2012, and the white voters who have joined the electorate are younger and likelier to support Mrs. Clinton than those who were already registered.