Here's a reality check for people spending a lot of time wondering whether Bernie Sanders's supporters will support Hillary Clinton if, as seems overwhelmingly likely, she is the Democratic nominee.

The most recent CBS/New York Times poll shows that 6 percent of Democratic primary voters (many of whom, as we've learned in this campaign, are not registered Democrats) say they won't support Clinton, plus another 2 percent who say they're not sure.

Back in late May 2008, a CBS/New York Times poll found that 12 percent of Democratic primary voters said they would vote for John McCain, plus 3 percent who said they wouldn't vote and 4 percent who weren't sure.

Obama needed, in other words, to overcome a hurdle that was twice as big as the one facing Clinton.

And for a sanity check, note that this aligns with other independent indicators.

Clinton got way more primary votes in 2008 than Sanders has gotten in 2016. Indeed, Clinton got way more primary votes in 2008 than Hillary Clinton has gotten in 2016. The 2008 race was much closer and also inspired more passion than this one.

And, of course, as of spring 2008, Clinton had the loyal backing of huge swaths of the institutional Democratic Party — major labor unions, many elected officials, a living former president of the United States — that signified a genuinely torn party.

Last but by no means least, McCain was a broadly popular figure with considerable cross-party appeal. He wound up losing pretty badly in the end due to a very unfavorable economic situation and a campaign that saw him abandoning most of his crossover positions. But as of late May it really wasn't totally crazy to imagine a hawkish, domestically moderate white Democrat feeling that he had a lot in common with McCain, especially if there was a little covert racial bias in the mix.

By contrast, while there are some real questions about whether Clinton can match Obama's turnout operation, there's no reason at all to think that people who sympathize with Sanders's critique of Clinton are going to suddenly go vote for Donald Trump.