In 2019, President Trump's prospects of winning re-election in 2020 seemed all but guaranteed. An April 28, 2019 Politico headline summed up why: "Dems sweat Trump's economy: We don't really have a robust national message right now." Of course they had no message. Trump's economy had gone gangbusters.
In Trump's economy, the poverty rate hit an all-time low. More Americans were employed than ever before. The unemployment rate fell to a 50-year low. This, in addition to the 7 million new jobs created. Middle-class family incomes had also risen more than five times higher than the previous eight years under Obama.
Trump had also triumphed over the myriad Democrat party hoaxes aimed at subverting the will of the American people and overturning the 2016 election results: the Trump-Russia collusion hoax and the failed quid pro quo impeachment, for example.
Leading up to 2020, the Democrats had already thrown everything plus the kitchen sink at Trump. They were empty-handed. Even as late as November 2019, the Democrats were "increasingly worried that their large and divided presidential field" didn't "have what it [took] to beat President Donald Trump" in 2020.
Democrats were desperate for something to campaign on. While they never found a robust national campaign message to campaign on, they did find a message to campaign against: COVID.