In 2008, the PUMA contingent of Hillary Clinton voters was quite persistent.

In June 2008, 20 percent of Clinton backers said they'd go for John McCain. In July, it was 22 percent, then 18 percent in August and 19 percent in September. It finally dropped to 14 percent in October.

But this time around, the voters who backed Bernie Sanders in the primaries are moving over to Hillary Clinton at a much more rapid pace.

Last month, 20 percent of Sanders supporters said they would back Trump over Clinton in the general election. This month, that figure is down to 8 percent.

The reasons for this significant difference are many. In 2008, Clinton was regarded as the more moderate candidate on the Democratic side. The press also described McCain as a moderate, though significantly to the right of Clinton. Apart from the warning sign of his selection of Sarah Palin as as his vice presidential running mate, the idea of a McCain presidency struck few people as a call to toss the nation over a cliff. Voters who had parted ways with Barack Obama during the rough and tumble of the primaries might have felt that John McCain would be, if not the better choice, at least not a disastrous one.

There may have even been a small contingent among Hillary 2008 voters disappointed enough at seeing a woman candidate come so close to winning the nomination only to fall just short, that the appointment of Palin was actually a slight balm. A few. Or maybe not.

In any case, declaring for McCain in 2008 might have contained a bit of a middle-finger to Obama, but it didn't embody the big screw-you to the nation that signing onto Trump's campaign involves in this season.