Medicaid had a big election night. Four states, all of them pretty red, had Medicaid expansion on the ballot. Three of those referendums won on Tuesday. In another two states that had refused the expansion, Democrats took the governorships. As Vox's Sarah Kliff noted, this could all result in as many as 500,000 Americans gaining health-care coverage.

This is important in and of itself. But it's also important for what it means for health care as a political issue. A big night for Medicaid means a big night for Medicare-for-all.

One of the big sticking points of the Bernie Sanders' proposal to expand Medicare to cover everyone is its political efficacy. Do Americans really want the government to provide health care? Many centrist Democrats think they don't and that more modest fixes to the current system are thus in order. But many leftists, including Sanders and an increasing number of 2020 candidates, think they do. In some ways, the midterms put their differences to the test.

To understand why, you first have to understand why Medicaid expansion is similar to Medicare-for-all.

Like Medicare — and, by extension, the Democrats' plan to expand that program to cover everyone — Medicaid provides people health coverage via the government. (Medicaid covers low-income Americans, Medicare covers elderly Americans, and Medicare-for-all would cover all Americans, period.)

But the funding nuances are also very important. Medicare is run entirely at the federal level (as Medicare-for-all presumably would be too) while Medicaid is run by both national and state governments. That shared pot complicates how Americans experience the program. Eligibility for Medicare is set at the national level: a relatively straightforward matter of your age and when you want to retire. Medicaid eligibility varies from state to state: Different state governments have different income thresholds for children, for adults, for pregnant women, etc. It can be frustrating.

Here's the thing: Medicaid expansions make Medicaid more like Medicare in this regard. Under ObamaCare's rules, states get more federal funding when they expand, but they also have to increase eligibility to basically everyone at or below 138 percent of the poverty level. The Medicaid expansion simplifies and broadens eligibility at once. So votes to expand Medicaid are votes to make the program more similar to Medicare — and by extension, a theoretical Medicare-for-all bill.

The other thing to understand is the Medicaid expansion's context within ObamaCare.

It actually started out as something of an afterthought. When Democrats first passed their health-care reform, what really excited them were the exchanges for individual plans. They understood it as a Republican-friendly approach that could win them bipartisan support. But they also really believed the exchanges would revolutionize health insurance and become the dominant way Americans got coverage. The Medicaid expansion was just a way to fill in the coverage gaps at the lower end of the income spectrum.

It didn't work out that way. Republicans turned on the law. More importantly, the exchanges were a political flop with Americans.

While the exchanges gave the individual plan marketplace more standardized rationality, premiums and deductibles remained painfully high. ObamaCare's subsidies were not nearly generous enough. Only the poorest Americans on the exchanges really got enough aid to make a sizable difference for their family budgets. And of course, shopping for plans on the exchanges was a hassle. Disapproval of ObamaCare topped approval by roughly 10 percentage points for years. And Democrats were punished by voters.

But in the background, the popularity of Medicaid and its expansion was gaining steam. As of 2013, only 23 states had taken the expansion. By the time election night 2018 kicked off, it was up to 34.

And in a bizarre twist, people on the exchanges actually resented and envied people on Medicaid, despite having higher incomes.

The exchanges were supposed to be a triumph of market-friendly technocracy. Instead, they became an afterthought. In terms of what voters actually cared about and where they saw real benefits for themselves and their families, the Medicaid expansion was the main event.

With Medicaid, there's no shopping — if you qualify, you get the coverage. And while states are allowed to impose premiums and cost-sharing in Medicaid, federal law severely limits those charges. This is actually a difference from Medicare, which tends to have a good deal more cost-sharing. Sanders' Medicare-for-all bill actually swings back towards the Medicaid approach and would pretty much do away with all cost-sharing. This is a point of controversy within the wonk community.

But Medicaid's experience demonstrates the political value of Sanders' choice. The program is a reliable, relatively hassle-free form of health-care security for Americans, despite not always having huge networks of doctors and providers.

Of course, the Republicans helped too, by focusing Americans' attention on what they could lose: The GOP's victories in 2016 brought the party within a hair's breadth of repealing ObamaCare and gutting Medicaid. (Those repeal efforts would've also killed the rules against denying coverage or overcharging people based on pre-existing conditions — another aspect of ObamaCare that is overwhelmingly popular with voters.) By mid-2017, ObamaCare's numbers had flipped, with approval outpacing disapproval by 10 percentage points.

That all brings us back to last night. Pro-expansion referendums won in Idaho, Nebraska, and Utah, although a successful referendum in Montana also scuttled that state's expansion. The Democrats took the governorships in Maine and Kansas, which eliminates the final barrier to expansion in those two states. Democrats won Wisconsin's gubernatorial race as well, by campaigning on Medicaid expansion, though they still must get past the state's GOP legislature.

Big picture: The Medicaid expansion has gone from covering fewer than half the states to covering almost four-fifths.

The lesson? When it comes to health care, voters value a program that is broad and simple. They want premiums and deductibles and other forms of cost-sharing to be minimized. And they'd rather avoid the grind of dealing with the insurance market. They just want security and reliability. That's the sort of policy they will reward at the polls.

That is, of course, a case for continuing to expand Medicaid. But it's also a case for Medicare-for-all, which would provide all those benefits to every single American voter.