As the presidential campaign has gone on, more Democratic candidates have warmed to the idea of eliminating the filibuster should Democrats win the White House and the Senate in 2020. When Elizabeth Warren suggested in February that she might support eliminating it if Mitch McConnell made it necessary, she was the first major candidate to go that far. Since then, not only has she said unequivocally that this is what Democrats ought to do, but, according to The Post’s latest tally, four more candidates have said they’d get rid of it (including Jay Inslee and Steve Bullock), and another 12 have said they’re open to the idea.

But the person leading all the primary polls, former vice president Joe Biden, is not among them. He reiterated that on Thursday when he told reporters, “Ending the filibuster is a very dangerous move.”

This might seem like some kind of arcane debate about parliamentary procedures, but it’s far more important. In fact, the fate of the next presidency and everything Democrats want to accomplish could depend on the answer to this question. While we spend a lot of time probing the distinctions between the candidates’ policies, such as whether they support Medicare-for-all or something more incremental, the filibuster question is even more vital for primary voters to understand and consider.

It’s not surprising that the Democrats running for president have been evolving on the issue. They’re no doubt hearing about it from activist Democrats, who are both fed up with Republican obstruction and eager for ambitious policy change. And the more they talk to voters about all the things they want to accomplish, the more obvious it probably seems that they won’t be able to do it with the filibuster in place.

And we should note how absurdly undemocratic the filibuster is in an institution that even on its best day gives disproportionate power to small states. It allows the 21 smallest states, which together account for only 11 percent of the American population, to veto anything the other 8 out of 9 Americans want.

But Biden, despite having watched Senate Republicans use the filibuster thwart Barack Obama’s legislative agenda for eight years, still has nostalgic feelings about the institution where he spent 36 years. And he has insisted ever since beginning his campaign that once President Trump is out of office, Republicans will wake as if from a dream, rub their eyes and join with him in a spirit of bipartisan cooperation. “With Trump gone you’re going to begin to see things change,” he has said. “Because these folks know better.”

Except they don’t. What they actually know is two things: They despise the entire Democratic agenda, and obstruction has been an extremely effective tactic for them, allowing them to thwart progressive change while paying no political price. They are not interested in bipartisan cooperation. Why would they be?

One person who understands this is Harry Reid, the retired Nevada senator who led Democrats in the Senate through 2016. He tells Sam Stein of the Daily Beast that climate change is an urgent crisis, and if Republicans stand in the way of addressing it, then “It is not a question of if. It is a question of when we get rid of the filibuster. It’s gone. It’s gone.”

Those who argue that the Senate should keep requiring 60 votes to pass most legislation make one argument most often: You might want to eliminate it when you’re in control, but you won’t be so happy about having done that once the other side takes the Senate back, which it inevitably will sooner or later. Which would make sense, were it not for some vital differences between Democrats and Republicans.

Senate Majority Leader Mitch McConnell (R-Ky.) hasn’t left the filibuster in place because of his deep respect for the traditions and norms of the Senate. It’s because he knows that it helps Republicans much more than Democrats. If it were in the GOP’s interest to eliminate it, McConnell would have done so the day after Trump was inaugurated. But he knew that he didn’t really need it. The Republicans had a very modest legislative agenda: Cut taxes for the wealthy and corporations, which could be done through the budget reconciliation process to circumvent the filibuster, and ... not much else.

They couldn’t even muster 50 votes, let alone 60, for their attempt to repeal the Affordable Care Act. And McConnell was probably relieved, since the health cataclysm repeal would have caused would have been a political catastrophe for his party. It’s a good example of how the party’s more extreme impulses can be held in check, and the failure to act on them can then be blamed on the filibuster. McConnell also knows that Democrats are the ones who have big, ambitious legislative goals, both because they’re the party that believes in government and because at this moment in history they’re seeking fundamental change.

So imagine it’s 2021, a Democrat has won the presidency, and Democrats have narrowly taken control of the Senate. That president, even if it’s a relative moderate like Biden, will have campaigned on promises of sweeping change: health-care reform that dramatically expands government’s role in providing coverage, strong action on climate change, steps to address the epidemic of gun violence, infrastructure spending, an increase in the minimum wage, increasing taxes on the wealthy, moves to secure voting rights, reform to drug laws, and so much more.

And if the filibuster remains in place, McConnell can say, “Sorry. You get nothing.” The next Democratic presidency will be an abject failure, both substantively and politically. All the goals, all the ambitions, all the visions for change could turn to dust while McConnell cackles with glee. So Democrats need to decide whether that’s an outcome they can live with.

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