Gregory Krieg and Tami Luhby / CNN

The fight to preserve Obamacare kept ambitious Democrats in near lockstep during the first two years of Donald Trump’s presidency. But as the party begins the process of selecting a 2020 nominee, the future of health care in America has emerged as an early flashpoint.

At the center of the early debate is whether Medicare for all — a national, government-run system — should be a centerpiece of the party’s platform. Beyond that, there is a more vexing question, given broad public support for the concept: how to both define what Medicare for all means and sketch out a realistic path to enacting it.

The ensuing disputes have frayed Democratic solidarity over health care, kicking off the first meaningful policy scrap of the 2020 primary contest. What had recently been passed off as minor or, given the Republicans’ grip on government, academic differences, are now opening up like seismic rifts — offering insights for voters not only into the candidates’ visions for health care, but on how they might act as president.