I think the Donald considers Harris his strongest potential opponent for 2020. For instance, Harris is black, which guarantees a high African-American turnout. Pointedly, Trump received just 12% of the African-American vote in 2016.
Hence, Democrats could pick up 100,000 to 500,000 votes and win in 2020 by running an African-American. If Democratic donors and strategists are smart, they will back Harris; or U.S Senator Corey Booker (D-New Jersey), and ignore everybody else.
Therefore, Trump wants Democrats to nominate a white candidate. Logically, he wants them to nominate the weakest white candidate.
I think the weakest candidate is Elizabeth Warren which is why Trump is Tweeting about her. Moreover, Warren is a great deal like Hillary Clinton.