Among those already voted Biden is 70.8%, Trump 27.5%
But Greenfield leads Ernst 66.3% to 29.1%
That's a bad sign for Greenfield that she is running behind Biden. But Biden is doing very well relative to the party registration of those who have already voted (DEM 52.6%, REP 30.3%, No Party 16.7% as of today).
608 k people voted so far which is 40% of 2016 turnout. If Biden is winning early vote that large then it is impossible for Trump to get a lead.
Well, mathematically it's possible. Trump just needs a really high turnout among Reps on the election day. But in 2016 the REP turnout in IA was several points higher that the DEM turnout. This year the Republicans may only hope that their turnout will be the same as the DEM turnout.