US Senators and 2020 Democratic Presidential Candidates Warren(MA),Brown(OH),and Sanders(VT) come
from states that currrently have Republican Governors. Baker(R-MA),DeWine(R-OH),and Scott(R-VT). In MA and VT, The Democrats have a filibuster/veto proof majority in the state legislature. The Democratic controlled state legislature in MA and VT can pass a law requiring the Republican Governor to appoint a US Senate replacement from the same party as the Incumbent US Senator.
Good thing Beto O'Rourke(D-TX) narrowly lost the 2018 TX US Senate Race because if he had won, he would not be mentioned as a 2020 Democratic Presidential or Vice Presidential candidate because TX has a Republican Governor and Republican control state legislature, The Republican Governor of TX(Greg Abbott-R) will appoint Beto's TX US Senate successor. Abraham Lincoln was the last defeated US Senate candidate to get elected POTUS two years after losing a US Senate Race.
A Warren/O'Rourke ticket will be more like the JFK/LBJ than the Dukakis/Bentsen-D ticket.