Generally speaking, Republicans are doing better in the polls when facing Sanders rather than Clinton. Using the Poll numbers from yesterday's PPP poll and today's Quinnipiac polls: there are only 4 of 18 match ups where Sanders does better. Two of those four are against Trump. (oddly enough, Sanders does relatively better against Trump than the rest of the GOP field in comparison to Clinton) Overall, in the last 2 days of polling, Sanders has been doing about 2% worse than Clinton. In PPP's national poll, Sanders does 4.5% worse. Sanders appears to be doing better at the state level than he is nationally.
Now, here is a quick graphic of the 2016 Democratic primary race. Clinton is blue and Sanders is green. The darker the shade means a larger margin. The lighter, of course, means the smaller the margin. The states in black have no polling data at this time. The point is, Sanders really only has 3 states where he has even a MoE lead. Only one of those has more than a single poll in the last 6 months. If Biden does not run, the path to victory for Sanders becomes even more remote.
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