More than a third of California’s active registered voters had cast their ballots in the recall election by Saturday, several days before polls close.

That hints not only at a fundamental shift in how Californians are casting their ballots, but when they are doing so, said Paul Mitchell, a vice president of Political Data Inc., a Sacramento-based supplier of election data.

“There will come a day where the highest voter turnout is not on Election Day,” he said.

That may not be this election, for which all of the state’s roughly 22 million active registered voters were mailed ballots. But trends suggest that the proportion of ballots cast on Tuesday, the last day of roughly a month of voting, will be much lower than in the past, Mr. Mitchell said.