During the 2016 presidential campaign, Donald Trump pledged to be a unifier -- bringing all Americans together. He hasn't even tried to do that. By supporting racists and white supremacists and xenophobes, he has further divided the country.
Now most or all of the 2020 Democratic presidential candidates are making the same pledge -- to unify this country. I don't think any of them would be a divider (like Trump), but any claim to be the one who can unify the country right now is just a pie-in-the-sky dream.
This country is badly divided right now -- thanks to the toxic leadership of Trump. And Trump's hard core base (racists, xenophobes, misogynists, religious bigots, etc.) are not about to unify with the rest of this country. They have bought into Trump's idea that Democrats (and the press) are their enemies. They have invested too much in hate to unify with anyone.
You could bring back the ghosts of great presidents like Jefferson, Lincoln, and F.D. Roosevelt, and they could not (together or individually) unify the United States right now. They might unify about 65% of us, but that 30% to 35% that love Trump are never going to willingly unify with the rest of America.
That's why I think it's silly for Democrats to even try to make the claim of being a unifier. They will be doing good to unify the Democratic Party (and I sincerely hope the eventual nominee can do that), but any hope of unifying the country right now is an impossible pipe dream -- and they know it.
Tell us why you want to be president, and what you'll try to do once you are there -- real things that you have a possibility of accomplishing. But don't tell us you'll unify the country. You won't. We know it, and so do you!