As of Thursday, there were 180 days until the election. Of the 540 days that made up the last 180 days of the past three races, the person who ended up winning led in 79 percent of them. There's some back-and-forth — but the person who wins has led pretty consistently. Clinton's lead in the most recent Real Clear Politics average, 6.4 points, was exceeded by past winners only 6 percent of the time. It's a big lead, in other words.
But, then, past experience suggested that Trump's lead in the primaries would collapse, and we saw what happened there.Bump rightly underlines the fact that this electoral cycle has had some unexpected twists, a feature that could apply to the general election, too.
By historical standards, it wouldn't be that surprising for the election results to swing further in one direction (either in Trump's favor or Clinton's favor). Elections prior to 2000 could often show considerable volatility.
According to Gallup polling of the 1988, 1992, and 2000 elections, the candidate who led in May ended up losing the popular vote in November. In 1988, Dukakis was up by around 16 points over George HW Bush in mid-May; Bush ended up winning by 8 points (for a 24-point shift from May). George HW Bush led Bill Clinton by about 6 points in early May 1992, and his son had a 5-point lead over Al Gore in May 2000. (Bush won the presidency in 2000 but lost the popular vote.)
The upshot of this is that there are a lot of possible models for an election--and there's no reason to believe that this race is settled.