Paul Krugman – a Hillary Clinton supporter, who should really start columns like the one in question with that caveat in bold text – wrote the not-so-cleverly-titled "Feel the Math" because he wants Bernie Sanders to stop competing in any serious way with Clinton, upcoming races in populous states like California and New York be damned! The whole thing is just a professorial stump-lecture for Hillary Clinton that is both arrogant and disempowering.
He starts off by acknowledging how surprisingly well the Sanders Campaign is doing, but then swiftly deflates the enthusiasm by pointing out that he doesn't really have a chance to win – maybe 10% chance – and so "it’s time [for professor Krugman] to lay out some guidelines for good and bad behavior" kids.
The rules he sets basically come down to: (1) You don't have to drop out, but you need to stop actually competing with Clinton, and (2) You need to throw some cash to the Democratic Party and other Democratic races.
I have less of an opinion about the 2nd point, but as someone who doesn't identify with the Democratic Party and who finds much of the Democratic establishment to be an impediment to real change, I'm not too concerned about Sanders keeping his fundraising close to his own campaign and not wanting to spend too much time or money on the Democratic party generally. I think a lot of people donating to his campaign feel the same way about their contributions.
The first rule is the one that I find most troubling, and really underlies a lot of the liberal elite's attitude about Sanders at the moment. It isn't just Krugman demanding that Sanders stop genuinely competing if he refuses to drop out, a top Clinton aide has said that Sanders needs to change his tone if he wants to get another debate with Clinton – a demand that amounts to the same thing. Even though Clinton has said that Sanders "stood with [the anti-Immigrant] Minutemen vigilantes" (a calculated lie), and she has falsely and angrily accused the Sanders campaign of lying about her contributions, and she has accused Sanders – who is arguably more pro-choice than Clinton – of not taking abortion rights seriously, among other distortions and accusations, it is time for Sanders to stop challenging Clinton in any way that may do damage to her campaign.
His central point is that Sanders is unlikely to win (because of the math) and so now he needs to play nice, even if Clinton doesn't. She, after all, has to prepare for the general election; so she has a different set of rules to play by.
But the fact is the race is far from over. A ten percent chance is still a decent chance – and probably one of the best chances a genuinely progressive/left candidate has had in a long time. (I would put his chances higher, though I recognize the steep, uphill battle his campaign has.) In states like mine, California, and other large, populous states like New York, Pennsylvania, Maryland and New Jersey, election day is still in the future. We are still having the debates, trying to convince our neighbors, co-workers, fellow students, about who they should vote for. There are enormous difference between Sanders and Clinton on foreign policy, money in politics, the environment, workers' rights, healthcare etc. There is also a clear distinction between the candidates' history and character – differences that should inform voters about who they should trust more. Sanders, in my opinion, is far and away the better candidate in all these regards; and the more voters learn about the differences, the more momentum his campaign gains. This is what Krugman and other Clinton supporters seek to stop when they trot out their math and their candidate's inevitable and likely victory.
The constant drumbeat from the corporate media, even from liberals in the New York Times and at MSNBC, about Hillary's inevitability has been Sanders' biggest obstacle (something the Clinton campaign has used as much as possible to her advantage). That perception of inevitability has been self-fulfilling – contributing to how well she has done so far, and thus adding to the sense that she is the likely nominee. Journalists and pundits amplifying that perception is a really undemocratic way of talking about elections. It is the opposite of how much of the establishment punditry have talked about the Republican nomination process, where there is constant hand-wringing about how candidates with far fewer delegates, but who are not Donald Trump, could still find a way to be the nominee (also a pretty undemocratic discussion but in favor of insurgent campaigns and quirks of party rules that could favor someone, anyone, but the frontrunner). In this sense Krugman, and much of those in the corporate media, play a role in disempowering voters and discouraging the substantive, democratic discussions that ought to be happening during this long primary season.
Of course, for Krugman, that is just fine as long as he is disempowering Sanders voters and quieting any issues that benefit his campaign at the expense of the Clinton campaign. Krugman's candidate is the inevitable winner (as far as he is concerned), so challenging her from the left is off limits. Sorry California and New York – maybe start organizing around the general election or something.