At this point in the primary campaign, he was ahead by ten points. Now, in one of the closer polls, he's down to Hillary by six.
Nor is it hard to see why. Trump only got about a third of the vote in the primaries. He succeeded because his opposition was so divided, and a relatively limited and marginal group of supporters was sufficient for him to win because it was significantly larger than the number of supporters any one of his opponents had.
But there is only one Hillary Clinton. Oops.
Perhaps the rest of us should be grateful for that. But it's the worst possible news for The Donald, whose relatively low voter ceiling isn't going to help him now that he can't rely on a divided opposition to artificially magnify his relatively limited support- especially since such a large number of Republicans are planning to vote for Hillary or for a third party candidate, or simply stay home rather than vote for a nominee who is utterly unacceptable to them.
It must be discouraging to be a candidate trying to win an election when only 29% of the voters see you favorably.