To listen to conventional wisdom, Hillary Clinton practically cannot lose the presidential election. The various forecasting services, from FiveThirtyEight to CNN to Predictwise, give the Democrat about a 70% chance of winning the White House in November. Few commentators are betting on Donald Trump. Yet the available evidence shows that the race is steadily trending toward Mr. Trump, whose victory remains quite possible.

Consider the polling trends. In a four-way race including Libertarian Gary Johnson and Green Party candidate Jill Stein, Mrs. Clinton’s lead is now only 2.4 points in the Real Clear Politics average. That’s down significantly from a month ago. In early August, following the Democratic Convention, Mrs. Clinton was up by six points in the YouGov/Economist survey, and eight points in the ABC/Washington Post poll.

When third parties are excluded, Mrs. Clinton does a bit better against Mr. Trump: She leads by 3.3 points in the Real Clear Politics average. Yet that figure has been cut by more than half in a month. In addition, the head-to-head matchup loses relevance each day that public dissatisfaction with the two major-party nominees does not subside. Mr. Johnson, the Libertarian, has held steady for months at about 7% support in the polling average, and the Green Party’s Ms. Stein has stuck at about 3%.

The latest surveys look even more ominous for Mrs. Clinton. Virtually all of those taken in the past week show Mr. Trump ahead, tied, or trailing but within the margin of error. The new CNN/ORC poll, out Tuesday, puts Mr. Trump up by two. Rasmussen’s release last Thursday showed 40% for Mr. Trump and 39% for Mrs. Clinton. The Reuters/Ipsos tracking poll out Friday had the same figures. The L.A. Times/USC tracking survey shows a statistical tie. The latest Investors Business Daily/TIPP survey has Mrs. Clinton up by one, but the margin of error is 3.4 points.

What accounts for this tightening? On the most straightforward level, it seems that Mrs. Clinton is coming down from the bounce she received after the successful Democratic convention. But something else has changed as well. In the latest ABC/Washington Post poll, published at the end of August, her image hit a career low: 56% of Americans viewed her unfavorably and only 41% favorably. This is a significant slide from even early August, when the same poll had Mrs. Clinton at 52% unfavorable and 46% favorable.