Medicare for All isn’t a new idea. It may be defined, basically, as universal health insurance under a single government-run, taxpayer-financed plan. It would certainly alter, and probably limit, the role of private health insurance companies like UnitedHealth. Senator Bernie Sanders of Vermont has supported the idea for years. But it wasn’t much of an issue for investors because it never went anywhere in Congress.

Now, however, Mr. Sanders is the front-runner among the announced aspirants for the Democratic presidential nomination. What’s more, he appears to have moved the entire political conversation into territory that is exceedingly uncomfortable for health care companies.

When he introduced a new version of his Medicare for All legislation in the Senate on April 10, the stock market noticed that his co-sponsors included at least four Senate Democrats who are also running for president: Kirstin Gillibrand of New York, Cory Booker of New Jersey, Elizabeth Warren of Massachusetts and Kamala Harris of California.

It’s far too early to divine whether Medicare for All — particularly a version that bans or severely limits private insurance — has even a modest chance of coming into existence after the 2020 election. Even now, amid all the hoopla, the odds may not be propitious.

The current Democratic leaders in Congress — Senator Chuck Schumer of New York, the minority leader, and Nancy Pelosi of California, the House speaker — have not supported it. President Trump and Republican leaders in Congress have been demanding a smaller government role in health care, not a larger one. And the giant health care companies, which have enormous wealth and influence, are, for the most part, committed to blocking the idea.