Will the Democratic Party gift-wrap the 2020 election for President Trump by nominating a too-far-lefty such as Harris, Sanders, or Warren?

Don't count out Warren or Sanders.

Biden is the safest.

….but that is what I thought before of Hillary.

The old crystal ball is mum on that one, Stu. But no, I personally don’t see either of them gaining the nomination, least of all Ms Warren. 
I would also note that few would’ve predicted either of the last 2 Presidents being elected at the same stage of their respective cycles.

I would further caution against overestimating Trump’s prospects of re-election. Sure, the wimpy Repub partisans who couldn’t stop from being steamrolled by the now Liar-in-Chief, should dutifully line up behind him, but don’t discount the measure of how fed-up an increasing number of Americans are becoming with the dysfunctional clown circus in the current WH. Trump lost the popular vote last time, has done nothing to expand his support, has done nothing for the “flyover states” who got him elected, and has demonstrated nothing resembling job competence, legislative skill or professional temperament.
Not to mention the ever-accumulating wreckage of Trump’s manifest iniquities, the deceit and division, brought to light in near-daily accounts of those who have worked in direct contact with him. This week should be more of the same. 
Even if he is re-elected, perish the thought, it will be ugly. This post was edited by Don Barzini at February 27, 2019 6:59 AM MST

Bernie's age is also against him - yes, he's not much older than Trump, but unlike His Royal Orangeness, Sanders is far more likely to actually attempt to do the job and the stress of it might very well kill him. Look how haggard Jimmy Carter was after just one term - and he was considerably younger than either of them.
As for socialism, America isn't ready - the specter of Joe McCarthy still haunts the Capitol and anyone left of center is immediately branded a Commie.
I'd like to see a moderate who isn't a refugee from the nursing home run - like Tammy Duckworth. Bipartisanship has been lacking for too long, a decorated combat veteran wiith centrist policies might achieve it.

But among Democrats there seems to be majority support for such ideas.  Which they don't view as "socialist" but simply fair and compassionate to let the government run things.  Just the other someone on this site, not a young person,  stated there should be no private property.  Don't know who "Harris" is but Sanders cultivates the resentment of success vote while Warren specializes in the "artificial-equality-through-inequality" enthusiasm.  Against which Trump - basically still a Democrat - seems to be the moderate.  If he has not hurt himself by mouthing and tweeting off. 

Doubt anyone even knows who McCarthy was and I did not until I looked him up.  He mouthed off so much he made himself look ridiculous and lost his support.  Quite surprisingly Trump is the one accused of Russian influence, not Liz Warren or Sanders. Which goes to show you that old "commie" stuff means nothing any longer to most people.