One of the most common arguments from Bernie Sanders and his supporters is that he'd be a stronger general election candidate than Hillary Clinton against Donald Trump. And make no mistake about it: Though both Clinton and Sanders have led Trump in the polls, Sanders has consistently had a much bigger lead. But this fact alone doesn't imply that Sanders would be stronger against Trump than Clinton. In fact, it's entirely possible Sanders wouldn't beat Trump at all if he won the Democratic Party's nomination.
Clinton has been a national political figure for over 20 years, has universal name recognition, and has been subject to aggressive, focused attacks from Republicans since the 1990s. This began when she was First Lady in the 1990s, continued during her 2000 Senate campaign, intensified again during her 2008 presidential bid, and reached a fever peak during her tenure as Secretary of State. Republicans have been hitting her relentlessly during the 2016 cycle, too, despite the fact that she's not even the nominee yet.
Sanders has been the target of exactly one negative ad from Republicans this entire primary, compared to the 120 ads the GOP has run against Clinton. Before he launched his campaign, it seems very few Americans knew of Sanders' existence.
Republicans have basically avoided mentioning him, other than a few flippant remarks here and there, and because the press has always viewed Sanders as a heavy underdog, he hasn't received anywhere near the level of scrutiny that general election candidates typically receive. Even now, fewer Americans have an opinion about Sanders than of either Clinton or Trump.
The point is that if Sanders won the Democratic nomination, his poll numbers would almost certainly fall from where they are now. He'd be subject to a sustained, aggressive assault from the GOP, and as multiple studies have shown, negative attack ads work in driving down candidates' favorability ratings.
If Sanders were to win the nomination, Republicans would most likely depict him as both unpatriotic and possibly an enemy of the state. They'd remind voters every day that during the Cold War, Sanders sided with Communist revolutionaries in Central America instead of the U.S. government. They'd attack his economic policies as unrealistic and dangerous, and use the disastrous interview Sanders gave on policy with the New York Daily News interview as proof.
Or how about that creepy rape fantasy Sanders allegedly authored as a youth? It's been reported on but hasn't been anything close to a major news story. If he became the nominee, however, you can bet your last dollar that Trump would be waving a copy of it around at every rally as evidence that, in fact, Sanders is the candidate with problematic gender views.
The media, too, would put a spotlight on Sanders in a way that it simply hasn't yet. We'd see in-depth analyses of his Senate accomplishments (which are slim), his unclear relationship with his first son, and whether or not his wife was responsible for bankrupting a college in Vermont.
None of this is to say that Sanders would definitely lose in a general election; he could very well win! But we certainly can't trust these hypothetical general election polls to predict whether or not this would be the case. A general election campaign would affect Sanders much differently than it would Clinton, because Clinton has already been vetted by the media and attacked by Republicans as extensively as any politician in America, including President Obama. Sanders has not, and we can't trust his general election poll numbers until he's actually in a general election.