Let’s consider a hypothetical. What if Bernie Sanders became president? It’s hardly impossible. If he wins in Iowa and New Hampshire and then either in South Carolina or Nevada, he could well be the nominee with the momentum that would create. If Donald Trump or Ted Cruz is the Republican nominee, which seems likely, then given the utter detestability of the Republican ticket, Sanders could win.

So then what? What does a Sanders presidency look like? I doubt it looks all the great. I have two primary concerns. First, what is the most important thing a president does? Is it leadership? Is it giving speeches? Is it proposing legislation? No, it is none of these things. The most important thing a president does is make appointments to offices and judgeships. That’s because a) appointments are really important and b) presidents are highly constrained by Congress so their personal agenda has only a limited ability to be implemented. That’s less true for foreign policy, but then foreign affairs is not Sanders’ strong suit. As many have said, what Sanders says he will immediately change, he really can’t. The president simply does not control economic policy to this extent. I don’t get the sense yet that Sanders is really thinking about what a Sanders presidency would look like. The Obama administration made a big mistake in taking too much time to get appointments made in the first year and he had prime-time political operatives working for him. So this is a concern. Or I guess a cluster of concerns about Sanders’ ability to govern and prioritizing where he actually move toward the changes he wants to see.

But I think my bigger concern about a Sanders presidency is that his base would almost certainly abandon him within a year. The left has learned nothing since 2008. In that year, the left looked at Obama as its savior. This was ridiculous if you looked at his record, which was of course much more slim than Bernie Sanders in 2016. But Obama spoke inspiring words about hope and change that people very much wanted to hear. His campaign became not just another campaign but a social movement. The left believed Obama would bring about radical change in all phases of the government. Remember the hopes that maybe Obama would name Michael Pollan as Secretary of Agriculture and he would fix our food system? Those were heady times!

Of course once Senator Obama became President Obama and he had to compromise with his own cranky caucus, not to mention racebaiting Republicans, he was almost immediately abandoned by the left. By the summer of 2009, many on the left had given up on Obama. We saw what happened in the 2010 midterms. By that point, Obama was just another sellout mainstream corporate Democrat. Which might even be true to some extent, but a mainstream Democrat is all he ever was. Even on his mainstream positions though, he had to engage in deep compromises. Yet that still created the ACA, a very real policy achievement that makes Obama the most successful liberal president since Lyndon Johnson.

But the abandonment of Obama is not the only relevant example here. Because look what has happened with Bill DeBlasio. As I have discussed before, overall, DeBlasio has put together a pretty impressive record in his first two years of mayor. Not perfect, but not bad. And yet wide swaths of the New York left think DeBlasio is a massive sellout! I know that picking a random Huffington Post piece is shooting fish in a barrel, but this is actually a relatively mild condemnation of DeBlasio’s term.

Quite frankly, I am flabbergasted that de Blasio is delusional enough to start fund raising now (to dismal success) for reelection. Mr. Mayor, with all due respect, you need to actually accomplish something in your first term, and get the people to view you as a leader before you start aiming for the brass ring again. Your time is running out to actually make any kind of an impact… and we don’t find using your mayoral time to fund raise, relentlessly chase press, and strategize all day long about how you’re going to take down your opponent very amusing. Do you realize how much could get accomplished if these politicians spent more time actually working for the people, and less time trying to take each other down, plant “blind source” items, and go dirty dancing for donor dollars all day long?

Here’s another example:

A racist police commissioner. Bloombergian gentrification. Failure to eradicate horse-drawn carriages. The grievances were scattershot for the several dozen protesters circling outside the first fundraiser of Mayor Bill de Blasio’s re-election campaign tonight, but they were plenty furious. A loose confederation of liberal animal rights, anti-police brutality and pro-Palestinian activists united at the Sheraton near Times Square to send a single message: we do not want Mr. de Blasio, who won a 2013 election as a proud liberal, as our mayor anymore. “Bill de Blasio is a sell-out. Bill de Blasio is a fake progressive and we want to make sure he doesn’t even have a chance of being able to continue to do the things he’s been doing here,” said Josmar Trujillo, the leader of a group that aggressively opposes Mr. de Blasio’s police commissioner, Bill Bratton. “Whether it’s the addition of extra cops at taxpayer money, whether it’s privatization of NYCHA [New York City Housing Authority] land, whether it’s his real estate-friendly rezoning plans that are being opposed in community board after community board–all these things are basically reiterations of Bloomberg and Giuliani and he’s doing it under progressive cover and we’re here to call it out,” he added. At one point several activists, including Mr. Trujillo, stormed into the Sheraton’s ornate lobby shouting “hell no with the status quo, Bill de Blasio’s got to go.” Police officers eventually forced them out. For the activists, there was a sense of betrayal and disillusionment with a Democratic mayor who promised a break from the Bloomberg era, which shunned liberal rhetoric. The activists attacked the mayor for his plan to sell off public housing land to developers, his decision to hire 1,000 new cops and stand staunchly behind Mr. Bratton, whose “broken windows” theory of policing overtly targets nonwhites, according to some progressive critics.

I could go on.

There is no way the same thing is not going to happen to Bernie Sanders. His “revolution” won’t happen overnight. People will become disillusioned. And then they will start looking for the Next Progressive Hope who will save them through inspiring rhetoric that will cower the opposition. I mean, all DeBlasio had to deal with was the open revolt of the New York Police Department near the beginning of his administration. If only he had just gone the distance and abolished policing instead of compromised in order to save his ability to govern to New York, all progressive goals would be accomplished!

None of this is of course to say that the left shouldn’t criticize Obama or DeBlasio or Sanders or anyone else. But a Sanders presidency is going to be under attack from the start, even if Democrats regain a narrow Senate majority that, even outside of the filibuster, gives someone like Heidi Heitkamp or Joe Manchin tremendous power as the 50th vote. If Sanders’ allies turn on him for the inevitable compromising he will have to make on his goals, his presidency will truly be doomed.