That’s not really what’s happening, what’s happening is that soft republican voters who were undecided a few months ago are rallying around Trump. Four months ago Trump numbers were around 41%, they’re now around 44%, look at the IBD poll, in August Trump was at 42%, in October he was at 44% then 45% and now he is at 46%.
There is certainly some truth to that, and that is to be expected as well. Parties rally around their nominee as attention starts to shift to the election, and that is the case for Trump and the GOP as well. In reality it looks like a combination of both. The last 3 IBD/TIPP polls on RCP go like this:
Biden 53 - Trump 43
Biden 50 - Trump 45
Biden 48 - Trump 46
That is Trump going up a bit, but also is Biden going down a bit.
https://www.realclearpolitics.com/epolls/2020/president/us/general_election_trump_vs_biden-6247.html#polls