Procrastination is usually frowned upon, but in a narrowly divided presidential election, voters who have put off making a decision until the last minute could find themselves with the power to decide the future of the country.

Both campaigns and their outside allies have spent millions of dollars trying to find and reach the relative handful of dawdlers, late deciders and genuinely torn voters who are still undecided about how (and whether) to cast their ballots — precious needles in the…