You are probably familiar with this, but it’s useful to begin with a quick delineation of the state of play. The U.S. Senate is evenly split between Democrats and Republicans, 50 seats apiece. Should votes on legislation fall along partisan lines, a tie is broken by Vice President Harris. But that’s not very common because of the filibuster, a tactic that stalls any legislation until 60 senators agree to let it advance or, more commonly, it’s abandoned.

In order to avoid the filibuster, the Democratic majority (granted solely by virtue of Harris’s position) can try to pass a bill using the budget reconciliation process. Created as a way to quickly adjust the budget, it’s become a preferred vehicle for narrow Senate majorities to pass legislation without it being blocked by the minority. But there’s a critically important catch: Any changes made under reconciliation rules cannot increase the budget deficit past a 10-year window.

How is that determined? Well, Congress has numbers wonks in place to figure it out. Any legislation hoping to be passed under reconciliation is evaluated by the Congressional Budget Office and the Joint Committee on Taxation, twin nonpartisan bodies that perform these types of analyses. But this is necessarily imperfect; there is an element of estimation at play that can often prove to be incorrect.

To highlight this point, let’s consider one of the CBO’s most important products, its annual estimates of the direction of the federal budget. I looked at the latest projections for the budget in each year since 2006 and plotted out how the CBO predicted revenue (money coming in) and outlays (federal spending) would shift over time.

Those projections changed a lot.

Here’s the static version. The shaded areas show the range of estimates in spending and income over the 10-year windows the CBO considers. (The ranges at either end are small because there are fewer data points, not because the estimates were more narrowly targeted.) For 2021, for example, the CBO estimates of revenue ranged over $2.5 trillion between 2011 and now. The estimates of outlays covered a slightly smaller range.

The point is not that the CBO is bad at its job. Things happen! In the animation, you can see the effects both of the recession that began in 2007 and the pandemic that began last year. Neither of those could have been worked into estimates a decade prior. And then there are the policy decisions themselves, changes in spending that are not projected well in advance.

Instead, the point is that there is obvious uncertainty (albeit more narrow uncertainty) in what the components of a bill will entail. The Biden administration has used its own projections to suggest that its reconciliation proposal will be revenue-neutral, a claim that has been a subject of mockery by President Biden’s opponents. (Those same opponents were generally quiet when the Trump administration made the less-defensible claim that its tax-cut bill would not increase the deficit.) But those projections are, like the product of the CBO and the JCT, informed guesses (in this case, nonobjective ones) about what the effects of the legislation would be.

In other words, we suddenly have a scenario in which a bill that might pass on a party-line vote needs to conform to third-party estimates of its economic impact simply so that the vote can happen. It’s hard not to see that as an aberrant way of passing legislation.

Of course, that bill probably wouldn’t have passed on a party-line vote anyway. It depends on two Democratic senators, Joe Manchin III (W.Va.) and Kyrsten Sinema (Ariz.), who’ve publicly opposed the scale of the reconciliation bill. Had Biden not embraced an effort to pull out infrastructure investments as a separate, bipartisan piece of legislation, it’s not clear where the vote might have landed. But now, armed in part with the limits imposed by the reconciliation process, Manchin and Sinema have spurred a new focus on the bill’s top-line cost — one that’s become a central focus of the political discussion.

On Wednesday, CNN released new polling looking at views of the reconciliation proposal. The poll, conducted by the firm SSRS, found that three-quarters of Democrats think that the party should push forward with all of the social safety net and climate change provisions that were originally proposed by Biden. Overall, though, there’s a split; only 4 in 10 Americans hold that view.

Interestingly, there is some daylight for those arguing for less-expensive bill. Among Republicans, 45 percent of respondents said that they thought a scaled-back version of the legislation should pass or (to a much smaller degree) that all of the proposed components should be kept in the bill.

Another question asked by CNN, though, should dampen some of the excitement party leaders might have about Democratic support. When Democrats and Democratic-leaning independents were asked which group of legislators was doing more to help the Democratic Party, there was an even split between progressives pushing for more liberal programs and moderates looking to control spending. Even just among Democrats, the progressives had only a narrow advantage.

So this is where the party finds itself. Unable to simply pass legislation, it’s reliant on the evaluation of third-party groups to determine whether it can rely on a party-line vote. But even then, the policy needs support from Manchin and Sinema, whose objections around cost can be linked back to the reconciliation rules and that, by now, have been embraced by about half of the party’s caucus.

All without the filibuster even having been invoked.