On-line or automated voice polls hurt Clinton but do not help Trump. This is exactly counter to what one would expect if there was some group of secret supporters ashamed to admit that they will vote for Trump. If anything, the undecided voters break for Clinton when pushed in a live phone interview. Here's the story from HuffPollster's email.
POLLING MODE AFFECTS CLINTON’S NUMBERS, NOT TRUMP’S - HuffPollster: “In telephone polls conducted by live interviewers, Clinton averages an 11-point lead over Trump in the HuffPost Pollster average. She’s approaching 50 percent, whereas Trump is struggling below 40 percent. These polls are averaging only 5.5 percent undecided and 4.4 percent for other candidates. Polls conducted either online or using automated voice technology over the telephone, without a real person on the other end of the line, tell a different story. In these polls, Clinton averages just under 44 percent support, a 5-point drop from the live telephone polls, while Trump loses less than 1 percentage point, still sitting at about 38 percent. That 5 percent who supported Clinton in live telephone polls, but not in these other polls, seems to have gone to the ‘undecided’ column. In online and automated phone polls, undecideds increase by 5 percent, growing to over 10 percent of voters….Since it’s Clinton’s support that increases when undecideds are pushed in live telephone surveys, the polls indicate that undecided voters tend to lean more in Clinton’s direction.” [HuffPost]