If you’d asked me a year ago whether a bill could pass Congress over the objections of the National Association of Realtors, the National Association of Homebuilders, the AARP, police unions, hospital associations and the AMA, and the higher education lobby, I’d have been deeply skeptical.

But more than that, I'd have thought I understood the circumstances under which a powerful coalition of interest groups could be defeated.

The legislation would need to be some combination of bipartisan (so it could survive defections), popular (so members would feel confident in bucking interest groups), and backed by some kind of clear mandate (so members would be uncomfortable defecting).

The Republican tax bill is none of those things. There's no indication that any Democrats will vote for it, and Republican leaders never really bothered to try to round any Democratic support up. The tax plan polls poorly. And far from reflecting a clear GOP electoral mandate, it violates Trump's campaign promise to avoid cutting taxes for the rich and violates Trump's campaign promise to deliver a tax cut to the middle class.

Of course the plan may yet fail to pass. But it seems clear that the bill's main problems in the Senate GOP caucus have basically nothing to do with either unpopularity or interest group politics, but instead hinge on a few members' concerns about the deficit and a few other members' concerns about whether it does enough to help owners of pass-through businesses.

If you think it’s a bad plan, then the relative inefficacy of interest group opposition will strike you as a bad thing. But my point today is simply that it's an important thing.

And more to the point — it's a new thing. It's a thing that violates the shared understanding of sophisticated journalists and political scientists of “how Congress works.” It's impossible to move unpopular, party-line legislation in the face of meaningful interest group opposition. So impossible that back in 2009, the Democratic majority rather famously didn't even try.

Not only was there a lot of emphasis on an ultimately failed effort to secure GOP support for the plan, there was overwhelming emphasis on ensuring that the key health care industry stakeholders were on board with the program. Even the ultimately abandoned House legislation that featured a public option insurance companies didn't like was a very watered-down form of public option to avoid raising hackles from doctors and hospitals.

Had Democrats gone down a different path and pushed a bill with a strong public option with payment rates linked to Medicare, we would have seen a very different health policy trajectory over the past few years.

Premiums would have been lower, which would have meant federal subsidy outlays would have been lower, which would have made it affordable for Congress to make the subsidies more generous. Enrollment in ACA exchanges would have been higher; there would have been no issue with “bare counties”; and, because of lower premiums, the “just pay the fine” option would have been less attractive, leading to more stable risk pools.

What's more in that world, the tensions between “Medicare for All” and “Improve the Affordable Care Act” as Democratic Party paths forward would have been much smaller, and that has a whole bunch of consequences of its own.

But from the vantage point of 2009, the whole idea was totally unthinkable. Even the public option proposal that got bargaining away wasn't anywhere near that ambitious. The need for bipartisanship or interest group buy-in — or both — was just taken for granted by everyone, even the House Progressive Caucus types who pushed the public option hardest. Republicans seem to be showing us this year that things are changing on Capitol Hill, with potentially huge consequences not just for their own bills but for the whole universe of legislative possibility.

This is an abbreviated web version of The Weeds newsletter, a limited-run policy newsletter from Vox’s Matt Yglesias. Sign up to get the full Weeds newsletter in your inbox, plus more charts, tweets, and email-only content.