Why do you think he's sinking? He's been at about the same level for several months now.
View attachment 297474
RealClearPolitics - Election 2020 - 2020 Democratic Presidential Nomination
From 40% in May/June to 28% now. Sanders was second at 15%, 25 points behind and today he is at 19% and down by only nine. Maybe Biden is not flailing but most certainly he is losing voters.
Given his record, think of how many gaffs he's going to make between now and the election if he is the nominee.
That's why I love Biden, he is a potential laugh when he opens his mouth.