But in recent weeks, it's also become clear they were very unlikely to do that.

In fact, two polls showed Sanders supporters rallying to Clinton rather quickly and overwhelmingly, with more than 8 in 10 saying they would support Clinton in the general election. In both polls, fewer than 1 in 10 Sanders backers indicated that they would support Trump — despite his efforts to woo them.

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A Washington Post-ABC News poll late last month showed just 8 percent of Sanders backers going for Trump — down from 20 percent the previous month.

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Similarly, a Pew Research Center poll released last week — and conducted around the same time as the Post-ABC poll — showed just 9 percent of Sanders backers going for Trump.

By contrast, after a hard-fought primary between Clinton and Barack Obama in 2008, Post-ABC polling showed a much-slower rallying effect. In June of that year, 20 percent of Clinton backers still said they would vote for Republican John McCain over Obama.

That number remained in the same ballpark until October, just before the election, when it dropped — but only to 14 percent.

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What's more, there are fewer Democratic voters dithering in 2016 than at this point in 2008. While in June of 2008, just 58 percent of Clinton voters said they would go for Obama, today that number is 81 percent. In other words, in 2008 a whopping 42 percent of Clinton backers hadn't supported Obama by June; today, just 19 percent of Sanders backers aren't on board, according to Post-ABC polling, and just 15 percent aren't on board according to Pew.

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So were Sanders backers just not that committed to opposing Clinton? Have they lost the will to fight? What's more likely is that Trump simply isn't a viable alternative to them. And in fact, their support for Clinton might be as much about opposing Trump as supporting a Democratic nominee they don't love.

The potential for a big crossover vote made much more sense in 2008, after all. In that race, Clinton was the more moderate Democratic primary candidate, and McCain was a moderate Republican nominee.

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In 2016, though, the idea that more liberal Sanders backers would go to Trump was always rather theoretical. Trump has offered some of the same populist and anti-establishment rhetoric that Sanders supporters eat up, but he has also staked our far-right positions on issues like immigration that don't appeal to such young and liberal voters. Sanders himself has said his supporters wouldn't support a "bigot" like Trump.

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And Sanders voters in general are simply more suspect of the Republican Party than Democratic voters as a whole. Pew data showed that Sanders's backers were more likely to be "afraid" of the GOP. About two-thirds (68 percent) describe themselves that way, versus 55 percent of Clinton primary backers.

There is one caveat here, and it's this: If a viable third-party option emerges — either Libertarian nominee Gary Johnson or Green Party candidate Jill Stein (who has offered her party's ballot line to Sanders) — that could be a destination for Sanders supporters who don't embrace Clinton. But even in that case, it's likely only to matter on the margins.

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A Bloomberg poll conducted earlier in June (June 10-13) than the Post-ABC poll (June 20-23) and the Pew poll (June 15-26) showed less of a rallying effect for Clinton — perhaps because it was conducted just after a hotly contested final day of the primary calendar, June 7. At that point, 22 percent of Sanders supporters were going for Trump and 18 percent were going for Johnson.

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The Post-ABC poll also showed an openness to third-party candidates. Including them in the mix caused 8 percent of Sanders backers to go for Johnson and 11 percent to go for Stein. That depressed Clinton's support among Sanders supporters to 65 percent.

This makes sense — again, in theory. Johnson and Stein are perhaps shorter ideological leaps for Sanders supporters. And given majorities of voters don't much like Clinton or Trump, there is certainly an opening there for Johnson and/or Stein to perform better than their party traditionally has.

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But I would argue the Clinton-Trump numbers are more important. Third-party candidates, after all, tend to poll better than they perform on Election Day, when voters want to pick a winner or vote against something they fear — which, to the vast majority of Sanders supporters, appears to be the GOP and Trump. The Libertarian Party's best-ever showing was 1.07 percent in 1980 (Johnson took 0.99 percent in 2012), and the Green Party topped out at 2.7 percent with Ralph Nader on the ballot in 2000 but hasn't cracked even half a percentage point since then.

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Even if some Sanders supporters do go third-party — which, again, may poll well early on but poll less well on Election Day — these more recent polls show the rallying effect to Clinton is already well ahead of the pace set in 2008. It will also likely continue.

And that's especially notable because all of the poll numbers from 2008 discussed above came after what's happening today happened in 2008 — when Clinton endorsed Obama. She endorsed him June 7, but a Post-ABC poll concluding June 15 showed her supporters going just 58 percent for Obama and 20 percent for McCain.