Biden and Sanders are well known. I don't believe the Iran mess will affect the polls much one way or the other; people have pretty much made up their minds between a Democrat and Trump.
I find it interesting that Biden leads Trump by 7, while Sanders leads Trump by just 4, in a state relatively less favorable to "free trade".
Biden up by 7 is a good sign. Trump polls low ("the elephant in the room"), but even if Biden wins it in Nov. by just 1, that's enough. I'm not as comfortable with Sanders' 4-point lead.
I will give a mathematical example, which should be right up your alley given your username. It is a simplified example so that the math doesn't get overly complicated, using hypothetical poll #s to illustrate the point that Sanders could well be more "electable" and actually do better than Biden even if Biden polls 3 points better than Bernie and even if a poll having him doing 3 points better is completely accurate.
The point is not necessarily that Sanders is more electable (maybe he is, maybe he isn't, my point is not to take any firm position on that), the point is just that you can't conclude too much from these poll comparisons, because they ignore some crucial factors (namely the influence of turnout).
Let's say a poll has Biden +1 and Bernie -2 against Trump (so Biden is nominally 3 points stronger in terms of electability).
Underneath the hood, you have the following #s leading to these toplines:
Biden:
wins young voters 70-30%
loses old voters 42%-58%
Bernie:
wins young voters 75%-25%
loses old voters 38%-62%
However, the crucial assumption made by the poll is that in both cases, regardless of who the candidate is, that the same electorate will turn out to vote. In both cases, the assumption is that 70% of the electorate will be made up by old voters, and 30% by young voters.
If you multiply out those numbers by the 70-30 vote share (old-young) split, you end up with:
Biden 50.4% - Trump 49.6% (Biden ~ +1)
and Trump 50.9% - Bernie 49.1% (Trump ~ +2)
However, in reality the vote share of young vs old voters is not invariant to who the candidates are. If Biden is the nominee, it is plausible that young voters (and also some other groups not taken into account in this simplified example) may be less enthusiastic and less likely to vote than they would be if Bernie were the nominee.
For example, suppose that with Biden, young voter turnout were less, such that 72% of the electorate were old and 28% young, whereas with Bernie, young voter turnout were higher, such that 67% of the electorate were old and 33% young.
Multiply out the support #s by the turnout, and you get the following:
Biden:
70% support from young voters multiplied by 28% vote share + 42% support from old voters multiplied by 72% vote share = 49.84% total support (Trump gets the remaining 50.16%).
Bernie:
75% support from young voters multiplied by 33% vote share + 38% support from old voters multiplied by 67% vote share = 50.21% support (Trump gets the remaining 49.79%).
So in this case case, despite the poll being entirely accurate in terms of the % support that both Bernie and Biden get, and despite Biden doing 3 points better than Bernie in the poll's matchup against Trump, in reality Biden ends up losing and Bernie ends up winning because of turnout differences that are not taken into account by the poll, since the poll (incorrectly) questions the same voters regardless of who is the candidate, thereby assuming away any variation in turnout/enthusiasm between candidates.
