If there is one point of unification among members of the Democratic Party in the waning days of the nominating process, it is that they very much would not like to see Donald Trump as president of the United States. That's not universally true; there are certainly some Democrats who would prefer Trump to Hillary Clinton or Bernie Sanders, just as there are some Republicans who'd rather see Clinton as president. But in a moment when universal consensus on anything seems essentially impossible, unity against Trump is probably as close as Democrats will get.
Which is why a central plank of Sanders's closing argument to Democratic superdelegates is that he consistently does better against Trump in general election polls than does Clinton. Sanders has argued that his strength against Trump is a key reason to buck Clinton's delegate advantage in Philadelphia and hand him the nomination, suggesting that he has a more certain chance of preventing a Trump presidency scenario that keeps liberals from sleeping well at night.
It's incontestably true that Sanders consistently has done better than Clinton against Trump in head-to-head polling over the course of the campaign. In live-caller surveys since last July in which Trump was pitted against both of them, Sanders has done better than Clinton in 33 of 37 polls — and in every one conducted this year. What's more, the difference between Clinton's margin over Trump and Sanders's has grown larger recently.
You can see that difference grow as the distance between the two lines below. Each approximates the margin of victory the Democrats have enjoyed over Trump over time.
If we average the margins for each month of the year, you can see that Sanders's lead over Trump was about six points larger than Clinton's in January, five points larger in February and six points larger in March in April. Last month, it was about 8.5 points larger, as the race between Trump and the Democrats tightened.
So far, this hasn't convinced any superdelegates to ditch Clinton for Sanders. Why not?
For one thing, Clinton still leads Trump in most national polls. As we noted on Wednesday, after tightening in the wake of Trump's solidifying the nomination, Clinton has regained the lead in RealClearPolitics' average of polls. In other words, Democrats probably don't have much reason yet to panic over Clinton losing the race.
For another, part of the reason that Clinton does worse against Trump is because of Sanders supporters. The Upshot's Nate Cohn made this point last week, noting that recent YouGov polling showed Clinton leading Trump by 40 points among Sanders voters — and Sanders leading Trump by 70. Trump has benefited from consolidation of Republican support; it's fair to assume that Clinton will similarly benefit once Democrats unify behind her candidacy.
This may also help explain why Sanders did so much better against Trump in May, as the tensions between the two Democratic camps increased. In Post/ABC News polling in March, a majority of Sanders supporters viewed Clinton favorably. By our May poll, a larger majority viewed her unfavorably. It also probably helps explain why Sanders does better with independents against Trump than does Clinton, since Sanders earns so much of his support from that group.
Then there's the question of how Sanders would hold up to a robust attack on his candidacy. Clinton argues that Sanders has never "had a single negative ad run against him." This isn't the case, but it's certainly true that Clinton has been attacked much more robustly and frequently. It's disadvantageous for Clinton to go after Sanders very hard, since she's trying to woo his supporters to her cause once the nomination contest is settled. And it's advantageous for Republicans to lay off Sanders, since keeping him in the race will keep Clinton fighting on two fronts. If Sanders earned the Democratic nomination, the relative calm he's enjoyed so far would change quickly.
It's to Sanders's continued advantage that he can point to these polls and his opponents can only point to speculation about what might happen if he actually got the nomination. But it probably doesn't help him that the superdelegates are a group of people who, essentially by definition, have been involved in politics a long time. It's a group that is more loyal to the Clinton team than to the Sanders team, clearly, but it's also a group that is more likely to understand the caveats that accompany Sanders's polling argument.
For superdelegates to tip the scales on behalf of Sanders would require something so substantial that it would validate ignoring the fact that Clinton has a wide lead in the popular vote. Polling conducted five months before the election probably won't do it, no matter how good the polls look.