I am losing faith in polling very rapidly. I took the Trump victory as just MSN/Fox manipulation; 538 gave him a 30%, and winning with those odds is pretty much possible.
But looking at the vast disparity between the environment I see and the forecasts before and after the early primaries, I don't understand how anyone thought Biden had a chance, nor how he can have one going forward. But the polls are saying that the massive front-runner with a huge base and an energized support network has a less than 50% chance of winning, about the same odds as a contested convention.
This is bizarre. Why didn't Biden face a contested convention when he was the "front runner?" The next few contests are going to prove it. If these forecasts keep being so far off the mark so regularly, I don't see how political forecasting can claim to be any kind of science.
Those forecasts are based on who's winning
at the moment. That can change rapidly as a result of such things as debate results, ad campaigns, or even more unpredictable things like the time Hillary collapsed or Trump's bus tape. It's impossible to predict the exact odds of a candidate winning from only the available data, since the real world is prone to unexpected changes. For example, maybe Bernie Sanders will suddenly die of natural causes before the primaries are over. Or Trump could die, and the Republican primary suddenly becomes a real contest. Those are both relatively unlikely, but they're both actual possibilities.
Basically, Biden had a much better chance of winning before the primaries began, before the debates and the first three actual elections. However, now that they have, Biden's chances have plummeted.
As for the contested convention, it was relatively unlikely when Biden was a clear frontrunner. However, now he isn't, the race is a lot more chaotic, and there's a decent chance that, if nothing changes, Bernie could win the primaries without getting more than half of the available delegates, since he lost too many elections to enough candidates that no single one has more than him.
It should be remembered though, it's really unlikely that nothing will change.