If you're a liberal, the left can be a real pain in the neck.

That's what many Democrats are probably now feeling as their party embarks on a period of intense internal policy debate. We've moved so far on so many issues, they'd say — why won't you stop complaining and give us some credit? Can't you take yes for an answer?

That, however, is not how it works. And that's probably a good thing.

I'm describing the following series of events: The Democratic Party has a generally agreed-upon position on an issue, which the left says makes Democrats a bunch of neoliberal shills, corporate sellouts, weak-kneed compromisers. Then, for a variety of reasons, the party moves to the left, adopting a significantly more liberal position. At which point the left says, "You're still a bunch of neoliberal shills." Which drives the Democrats nuts.

Supporters of Bernie Sanders (and Sanders himself) are part but not all of "the left" I'm referring to, for lack of a better term. They tend to be opposed to any half-loaf thinking, believing that partial victories are just defeats by another name. And they are not particularly encouraged by what they see in today's Democratic Party, or at least aren't inclined to say so publicly.

That's despite the fact that by any sane measure, the Democratic Party has not only moved significantly to the left in recent years, it continues to move in that direction on an almost daily basis. The party is more supportive of immigration than ever before. It's advocating more aggressive gun control than it has in years. It's just about ready to favor legalization of marijuana. Nearly every Democrat wants a $15 minimum wage. Just about every 2020 Democratic presidential candidate now proposes new plans to achieve universal coverage through expanded Medicare or Medicaid. They're now seriously debating a federal job guarantee, a proposal so radically socialist that Finland — Finland! — is abandoning its experiment with one. As my colleague Jeff Spross recently wrote, "Quietly but steadily, the Democratic Party is admitting that Sanders was right."

To all that, leftists tend to reply: Not good enough.

Nowhere is that more clear than on health care, where there has been a radical shift in Democratic thinking, away from simply strengthening the Affordable Care Act and toward a dramatically expanded role for government. But to many on the left, it's "Single Payer or Bust," as the title of a recent article in Dissent put it. "Any Democrat worth their salt that doesn't unequivocally say Medicare-for-all is the way to go? To me, there's something wrong with them," said former Ohio state senator Nina Turner, a prominent Sanders supporter. While there are multiple systems in the world that achieve universal coverage at reasonable cost but are not true single payer, any proposal that retains even a small role for private plans, no matter how tightly regulated, is nothing but watered-down establishment policy gruel.

But you know what? That's fine. Everyone to the left of center doesn't have to agree on policy, and it's better if they don't. Make your case for single payer, and I'll happily make the case for the system I'd favor. Eventually Democrats will arrive at enough agreement on what their next health-care reform should be so that when they have the chance, they can try to put it into legislation. What happens between now and then is a debate, and in that debate it's important to have some people take a position farther out than the party will ultimately be willing to go. They can raise important points about fundamental principles, and force those more toward the center to provide strong defenses of the plans they've come up with. The more robust that debate is, the stronger the policy will be.

It isn't even a problem that many of the leftist critics aren't actually part of the Democratic Party. Bernie Sanders himself isn't — he's an independent, and stayed that way even when he ran for the Democratic presidential nomination. He and other leftists feel much more at home challenging the party from the outside. As I've argued before, there won't ever be a point at which they say, "Good job, Democrats — now you've given us what we asked for, and we're satisfied," because the belief that the party isn't liberal enough is a core part of their political identity. That's not to mention the fact that once you've said the party has gone far enough left, you lose some of your ability to pull it even farther.

Any party or political movement needs both pragmatists and idealists. It also needs people who will question its policy consensus, even if they do so in uncomfortable, occasionally maddening ways. A time like this — when the Democrats are out of power and are formulating their agenda for the next time they have the ability to implement change — is the best time to have those arguments.

The idea that the left is winning may be anathema to those who derive their purpose from being underdog revolutionaries always battling the intransigent powers-that-be. But everybody has their own role to play. And for some people, refusing to take yes for an answer is the right thing to do.