The Kamala Harris effect on the polls has been dramatic

But throughout the intervening period polls showed that the Democratic Party was weathering the storm despite its standard-bearer taking on water. When given a choice between Mr Trump and an unnamed Democratic candidate, rather than Mr Biden, Democrats enjoyed a clear lead. Who was this mysterious Democrat who could both excite the party’s base and work to woo moderate and undecided voters? It seems that the answer may have been hiding in plain sight: Kamala Harris, who became the presumptive nominee in the days after Mr Biden stepped aside.