Trump has been the Republican front runner for the primaries for many months now, so that is nothing new...
I look at it this way.
In the Iowa caucus, fractionally more than half the very committed members of the Republican party (the ones who were prepared to go out in the evening of a really freezing day) voted for him; that means almost half the Republicans who felt strongly enough about it to vote at all, voted against him.
Of the ones who voted against him, about half have stated they will never vote for Trump again. Ten per cent have actually said they will vote for Biden if Trump is the Republican candidate.
If he gets convicted of any of the 91 crimes of which he is currently accused, almost 30% of all Republicans polled have said they will then not vote for him.
I doubt he will pick up the independent voters he needs to win, and I very much doubt he will get enough Democrats voting for him to make much difference. So he has a whole lot less than half the vote, and may get a considerable drop in the numbers who vote for him even in the ultra-red states. The Democrats may not have enough to win there, but they may well have enough to come very close to doing so.
(There is an astonishing number of flat-out lies about Trump's performance as President on this thread! If I were polite I'd say that people may be mistaken and so on, but actually I think they are simply lies, being told deliberately to boost Trump.)
Oh, and for people saying how much better Sanders would be than Biden because Biden is too old: Biden born 1942, Sanders born 1941, what were we saying about senile old men again? (I have seen nothing to indicate that either of them is senile, and at least they both know what year it is and don't think we might get dragged into WWII if we're not careful, unlike at least one person I could name.)