Ron Brownstein: “Gillum, Abrams and O’Rourke are road testing the strategy that many liberals prefer for the presidential race: placing the greatest priority on expanding the electorate by turning out more minority and young voters rather than focusing on reassuring ordinarily Republican-leaning whites disillusioned with Trump.” “If any of those three win, it will be a great boost for 2020 Democratic candidates whose principal strength is their potential to mobilize younger and non-white voters.