You can’t beat somebody with nobody and right now there’s nobody in the Democrats who look to be a threat.
You saw what happened to Bernie Sanders (probably more honest than Hillary, but I am glad he is not POTUS). The big money Dems and CNN cheated him shamelessly in the primary, threatened and/or bribed him to support H, and then blamed him for even being a candidate in the primaries, since it was supposed to be H's coronation.
This highlights the difficulty of uniting what is left of the D party. Even if Tulsi Gabbard ran in the primaries and didn't get cheated, the warmongers in her party would want to buy her off or ruin her.
I don't know who they can find who would beat Trump.
Trump has always been the only person who could beat Trump, it was true in 2016, it’s true today and it will be True in 2020.