Senator Elizabeth Warren of Massachusetts and three other liberal presidential candidates support a Medicare-for-all bill, which would create a single-payer health plan run by the government and increase federal spending by at least $2.5 trillion a year, according to several estimates. But Ms. Warren’s determination to sidestep an essential but deeply controversial issue at the heart of the single-payer model — would people lose the choices offered by private insurance? — illustrated one of the thorniest dilemmas for several Democrats as the 2020 primary gets underway. (Jonathan Martin and Abby Goodnough, New York Times)