Fauci has served in several administrations for both parties. He is an expert on communicable diseases and tells the truth as the science determines it, including noting what we don't know.
Trump, on the other hand, is a hack. He clearly doesn't understand a lot of the basics (testing produces cases???). But he has a HUGE ego and doesn't like to be corrected.
So, when Trump says something stupid (which he does on a regular basis) and Fauci contradicts him with the facts, Trump lashes out.
In this case, he claims Fauci is a Democrat when, in fact, Fauci is not registered as being in either party.
But Trump is using this to malign Fauci, suggesting that Fauci is unreliable because he is bringing politics into this, when he *never* has before and, it is clear, isn't in this case either.
So, this is Trump being Trump: someone with the ego of a four year old, who doesn't understand the basics, trying to make fun of someone who knows what they are talking about.
Trump is trying to deflect his own mismanagement of this crisis by blaming someone else, whose recommendations Trump has consistently ignored, to all of our detriment.