One of the problems Hillary Clinton has had in this primary so far is deciding what, exactly, her case against Bernie Sanders should be. She's had a difficult time of it because, as I've written, even her own supporters generally like Sanders quite a lot personally.

As a result, she's been throwing a bunch of things against the wall to see what will stick. Sometimes she says Sanders is too left-wing, like on his health care views. Sometimes she says he's not left-wing enough, like on gun control and immigration. Sometimes she says she essentially agrees with him but that her plans for achieving their shared goals are more realistic. And lately she's been arguing, rather unconvincingly, that the problem with Sanders is that he's insufficiently supportive of Barack Obama.

But a campaign needs to define its opponent — to tell a simple story about why someone shouldn't vote for him or her. And together, all these arguments from Clinton end up sounding like a muddle. They don't paint one clear picture of Sanders — indeed, they often contradict each other. Is he too liberal, not liberal enough, or just as liberal as Clinton?

That's why, after a two-hour debate during which Clinton flitted back and forth between all the criticisms listed above, I was surprised to hear that in her closing statement she finally delivered a clear, easy-to-understand explanation for why Democratic primary voters shouldn't support Sanders. Here it is:

We agree that we've got to get unaccountable money out of politics. We agree that Wall Street should never be allowed to wreck Main Street again. But here's the point I want to make tonight. I am not a single issue candidate, and I do not believe we live in a single issue country. I think that a lot of what we have to overcome to break down the barriers that are holding people back, whether it's poison in the water of the children of Flint, or whether it's the poor miners who are being left out and left behind in coal country, or whether it is any other American today who feels somehow put down and oppressed by racism, by sexism, by discrimination against the LGBT community, against the kind of efforts that need to be made to root out all of these barriers, that is what I want to take on. And here in Wisconsin I want to reiterate, we've got to stand up for unions and working people who have been at the core of the American middle class and who are being attacked by ideologues, by demagogues. Yes, does Wall Street and big financial interests along with drug companies, insurance companies, big oil, all of it, have too much influence? You're right. But if we were to stop that tomorrow, we would still have the indifference, the negligence that we saw in Flint. We would still have racism holding people back. We would still have sexism preventing women from getting equal pay. We would still have LGBT people who get married on Saturday and get fired on Monday. And we would still have governors like Scott Walker and others trying to rip out the heart of the middle class by making it impossible to organize and stand up for better wages and working conditions. So I'm going to keep talking about tearing down all the barriers that stand in the way of Americans fulfilling their potential because I don't think our country can live up to its potential unless we give a chance to every single American to live up to theirs.

Regardless of whether you agree with Clinton here, I suspect this was a significant moment and that we'll hear a whole lot more of her and her surrogates attempting to portray Sanders as a "single-issue candidate." Because this one narrative accomplishes several of Clinton's political objectives:

It paints Sanders as a kind of protest candidate who's just in the race to make a statement and shouldn't be taken all that seriously.

It advances Clinton's argument that she has broader experience and qualifications on many more issues — that she's more serious than him.

It implies to women and nonwhite voters that Sanders just doesn't care about issues important to them all that much.

It portrays Sanders's diagnosis of what ails America — mainly the influence of big money — as simplistic.

It's a reason that Sanders shouldn't be the nominee, and it doesn't require people who like him (as many Democrats and even Clinton supporters do) to stop liking him.

And, unlike many of Clinton's other arguments against Sanders, it has the ring of truth to it — Sanders really does bring up Wall Street, corporations, and the wealthy in his answers to practically every question (in this debate he said he'd improve race relations by getting rid of "tax breaks to billionaires"). And he seems less comfortable when he discusses other topics.

That's not to say that this argument will work for Clinton. Sanders believes his simple message is the key to his success so far, and he may well be right. But in contrast to the potpourri of disparate criticisms Clinton has offered so far, it does have the advantage of telling a clear and at least somewhat plausible story about her opponent. We'll see whether she returns to it in the coming weeks — and, if so, whether Democratic voters buy it.

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