Historically, whenever the CPUSA makes its endorsement, right-wing demagogues use this as their opportunity to rehash the same canned take on how the Democrat is so radical that even the communists support them. The standard liberal response, of course, is to insist that the CPUSA is irrelevant and that its politics say nothing about the Democratic nominee. Seriously, this happens every single time:
There are only two kinds of people who think that the Communist Party USA's endorsement is evidence that the Democratic Party has leftist credibility: 1) right-wing demagogues who want to smear the Democrats as communists and 2) right-wing demagogues who want to try to co-opt communism's prestige and appeal. All that's changed is that communism's increasing popularity has made (2) more useful to the right.