What's the evidence? On-line polls have him stronger than person-to-person phone surveys. Steven Benen offers a theory of why that may be so.
... there’s speculation that some Republican respondents may be embarrassed to voice support for Trump when an actual person calls their home, but feel less awkward in online or robo-call polls, which might explain the gap.
And if that’s true, Trump’s support in traditional, independent, historically reliable polls may be understated a bit, because some of his supporters feel awkward about admitting it out loud.
Check out the evidence cited in Benen's report.