If somebody offered you a sandwich for free, you’d probably take it. $10? You might take it, if it was really good. $30? No thanks.

Voters are beginning to do similar cost-benefit analysis on Medicare for all, the Democratic plan for universal health care that’s sure to be a top campaign issue in 2020. In a January Morning Consult poll, 56% of respondents said they support Medicare for all, while 29% opposed it. Less than a month later, support fell to 50% while opposition rose to 38%.

The only thing that changed? People began to get a better understanding of what Medicare for all actually is—including the need to eliminate the private insurance system.

Democratic Sen. Kamala Harris, who’s running for president, triggered this deeper level of analysis on January 29, when CNN’s Jake Tapper asked her if she favored getting rid of private insurance. “Let’s eliminate all that,” Harris said. “We need to have Medicare for all.”

‘If you like your health care plan, you can keep it’

Americans feel no special love for insurance companies, but they’re not eager to give up what they’re familiar with, either. About 156 million Americans—nearly half the population—get health care coverage through their employer. Another 21 million buy their own insurance in the private market. So Medicare for all would move 177 million Americans from a health care system that generally works into a government-run system that has never covered that many people.

President Obama’s famous mistake with the Affordable Care Act was telling Americans, untruthfully, “if you like your health care plan, you can keep it.” At least 4 million people lost their coverage because it didn’t meet new rules under the ACA. Obama’s flub and the abrupt loss of coverage for those folks made the law unpopular from the start, and left it vulnerable to Republican efforts to kill it, which still aren’t over.

Medicare for all would end insurance-as-we-know-it for 44 times as many people as were affected by the ACA changes. Medicare is generally a well-run program that its participants like—but extending it to the entire population would entail massive changes likely to put the program under stress.

The biggest change would be a profound reworking of the tax code, since Medicare for all would require at least $3 trillion a year in new government revenue. So business and individual taxes would rise across the board. There would be offsetting savings, since companies and individuals would no longer pay insurance premiums and many out-of-pocket costs. But the transition would be turbulent and some people would end up as net losers.