Notice that “undecided” or “other” or “don’t know” isn’t a stated option? Live interview pollsters can do that, since the respondent is free to say “other” or “I don’t know,” and the interviewer can record it as such. In the NBC/WSJ poll, 9 percent volunteered “other” or “neither,” and 2 percent weren’t sure.

But this is a lot more challenging for an online pollster. If you put “don’t know” or “other” as an option, a lot of people are going to take it. But if you don’t include it, you have a problem: What will the truly undecided voters do? They could provide an unreliable answer or leave the survey. If you provide them the option to “skip,” they might skip through the whole questionnaire.

In the end, many of the online pollsters give voters the option to choose undecided. Voters are taking it in huge numbers.

You can see the effect in another type of NBC News survey: those conducted online by SurveyMonkey. At the beginning of May, the first NBC News/SurveyMonkey general election poll included an explicit “don’t know” option. Mrs. Clinton led, 43 to 37, with 19 percent selecting “don’t know.”

But in their second survey, conducted after Mr. Trump emerged triumphant, “undecided” was no longer an option. Respondents could go out of their way to skip the question and leave it blank, but they couldn’t simply say “undecided.” The result? Mrs. Clinton led, 49 to 44 percent, with just 7 percent electing to skip the question.

This doesn’t mean there isn’t a social desirability bias against Mr. Trump. One could imagine, for instance, that Mr. Trump would receive notably more support in the online surveys if they had as few undecided voters as the live-interview surveys.

But it does make it harder to compare the results of the online and live-interview surveys, since there’s no way to be sure whether Mr. Trump would still be doing better if there were fewer undecided voters.