Bernie Sanders won a decisive victory in Nevada’s Democratic caucus last Saturday, one made more impressive by the way it was accomplished: by mobilizing a multi-racial, grass-roots coalition around questions of economic and political justice. He followed this victory with an equally impressive speech emphasizing the importance of unity in the effort to both defeat Trump and improve the lives of “working families and the middle class.”

This does not obviate the real limits of the Sanders campaign and the challenges it must address moving forward, if Sanders is to successfully claim the nomination much less the presidency.

On Saturday night Jacobin, the organ of Sanders’s hard-core ideological supporters, declared that “After the Nevada Blowout, It’s Bernie’s Party Now” (indeed, the caption announced that “he’s on the way not just to the nomination, but the White House”). This represents less a “materialist” analysis than wishful thinking. There is a long way to go in the primary season, and a few early primary wins of a charismatic candidate does not a “political revolution” make.

All the same, whatever the vulnerabilities and risks of a Sanders nomination, it seems clear that right now he is the strongest candidate in the field, and that every other candidate presents greater vulnerabilities.

Last week I argued that if Sanders or any other candidate is to claim the nomination, they must do the political work needed to win a delegate majority of the party they wish to lead, whether on a first or a second convention ballot. I stand by this position. All of the candidates need to run their campaigns with the understanding that their primary opponents are intra-party competitors for the Democratic nomination with whom they will have to reach an accommodation, and are not political enemies. Donald Trump and his party are the political enemies, and it will be necessary for Democrats to come together if Trump is to be defeated.

There is an obvious temptation for every campaign, and especially its most enthusiastic activists, to forget this, and to press its advantage by virtually any means necessary. This is a dangerous temptation, and to succumb to it is politically irresponsible.

Unfortunately, Pete Buttigieg has succumbed. And by doing so, he has exposed his deep weaknesses as a candidate. He is smooth, polished — and also woefully inexperienced. Lacking in any real grass-roots political base, he is running primarily on the basis of his youth and his experience as the mayor of a Midwestern post-industrial city.

Just over a century ago Max Weber famously associated political responsibility with a mature and serious care for the consequences of one’s acts.

Pete Buttigieg lacks such maturity, and his Nevada speech epitomizes why.

Having come in third in the polling, he behaved as if he were the obvious winner, and the “future of the Democratic party” personified: he then focused his speech on an extended attack on Bernie Sanders.

Claiming to stand for a broad majority, Buttigieg asserted that “Senator Sanders believes in an inflexible ideological revolution that leaves out most Democrats, not to mention most Americans.”

Claiming to support “empowering the American people to make their own healthcare choices,” he asserted that “Senator Sanders believes in taking away that choice.”

Claiming to stand for “a tone of belonging,” Buttigieg asserted that the “Sanders revolution” has “the tenor of combat and division and polarization leading to a future where whoever wins the day, nothing changes the toxic tone of our politics.”

Buttigieg portrayed Sanders — who at the time of this writing won 45% of caucus votes, to his own 15% — as the strident, divisive, and narrow candidate, presenting himself as the true man of the people. “That is the choice in front of us,” he said sternly:

We can prioritize either ideological purity or inclusive victory. We can either call people names online or we can call them into our movement. We can either tighten a narrow and hardcore base, or open the tent to a new and broad and big-hearted American coalition. This is our shot. This is our only shot to beat Donald Trump. So, I am asking Americans to make sure that we get this choice right.

I am not suggesting that Buttigieg should refrain from competing. He wants to defeat Trump and imagines himself to be the strongest candidate for the job. At this early stage of the campaign, there is no reason to expect him (or any other candidate) to refrain from highlighting his differences from his competitors and even from criticizing them.

Indeed, some of the points Buttigieg made — especially about the tension between Sanders’s campaign rhetoric and the challenges faced by down ballot candidates — represent valid concerns (although for the life of me I don’t know why Buttigieg imagines himself to have especially strong “coattails”).

But in the face of Sanders’s strong showing in this demographically diverse state, and Buttigieg’s own lack of support among African-American and Latinx voters, Buttigieg might have proceeded in a less self-righteous and personal way.

More important, Buttigieg is all about trumpeting his “welcoming” campaign. And yet despite his almost metaphysical commitment to “civility,” his own rhetoric, in this speech and others, is extremely and deliberately divisive and polarizing — against Bernie Sanders and his campaign.

While Sanders’s Medicare for All Plan is surely not above reproach, it is a canard to state that Sanders simply wants to take peoples’ choices away.

And it is really deplorable to imply that the “tone” of the Sanders campaign, which is all about inclusion, simply mimics the tone of Trumpism, which promotes the rhetoric of a lynch mob.

Buttigieg’s attacks on Sanders might help to keep his candidacy alive. But they do so at the cost of slandering the party’s current leading candidate and his supporters as “one extreme faction.” That is his right. But by launching these attacks as his chances of becoming the nominee diminish, Buttigieg is making it less likely that the party will be able to come together behind a candidate in the fall campaign.

The primary contest is not over, and it will remain fractious for some time to come. But Sanders’s post-Nevada “victory” speech, unlike Buttigieg’s attack on Sanders, was exemplary in its graciousness and its appeals to unity against Trump. Sanders seems to be continually growing as a candidate. Given his leading status, it will be ever more important for this growth to continue — and for the other candidates to acknowledge it.

Jeffrey C. Issac is James H. Rudy Professor of Political Science at Indiana University, Bloomington.