Trump vs Biden : Biden leads 50/46
Trump vs Sanders : Sanders leads 49/47
Trump vs Warren : Tied at 48/48
Trump vs Buttigieg : Buttigieg trails 45/48
So far the January average is looking similar to the December average for Biden, 6 H2H reputable polls in December with an average Biden lead of 4.5%, 5 such polls in January with an average Biden lead of 4.8%.
Sanders led by 2.7% in average of December polls, leads by 3.6% in average of January polls so he does seem to have gained some ground vs Trump.
