How can President Biden get his $1.9 trillion pandemic and economic relief package past Congress’s slim Democratic majorities and the Senate’s 60-vote barrier for passing significant legislation? With Biden’s consent, congressional Democrats are turning to a procedural shortcut known as “reconciliation,” a budget tool that would prevent Senate Republicans from filibustering and blocking the Democrats’ plan from a floor vote.

Reconciliation has cleared the way for congressional majorities nearly two dozen times since it was created in 1974 and first used in 1980. But two challenges await Democrats. First, reconciliation comes with some tricky rules that could crimp Democrats’ plan. And second, unless Democrats can attract a few Republican votes, they will succeed only if all 50 people in the Democratic caucus “yea,” from liberal Sen. Bernie Sanders (I-Vt.) to more moderate Sen. Joe Manchin III (D-W.Va.)

Here’s what you need to know about reconciliation and potential pitfalls ahead.

What the heck is reconciliation?

Congressional Democrats enacted the Congressional Budget Act (CBA) over President Richard Nixon’s veto in 1974. That law gave Congress new power in federal budgeting, including the ability to map out a budget blueprint and to change federal laws to ensure Congress can meet its revenue and spending goals.

The law originally directed Congress to write two budget resolutions over the course of a fiscal year. The first resolution would propose an annual budget blueprint, which the second would refine. Budget resolutions set broad parameters for the annual federal budget, but they do not have the force of law. Instead, the CBA authorized an optional “reconciliation” bill that would make the actual changes in law to bring federal revenues and outlays in line with the second budget blueprint. For example, if the budget blueprint called for more Medicaid spending to provide health care for the poor, lawmakers might use the reconciliation bill to revamp the federal tax code to help pay for the increased health-care spending.

Congress rewrote the budget law in 1985 to eliminate those second budget resolutions. But reconciliation remained on the books for changing federal laws to meet congressional budget goals. Lawmakers originally viewed reconciliation as a tool for reducing federal deficits, but Congress over the years has stretched it in ways that often increase the deficit. Most recently, the GOP-led Congress in 2017 exploited reconciliation to cut corporate taxes, estimated to cost the government nearly $2 trillion in revenue.

Why can’t reconciliation be filibustered?

Most legislative measures in the Senate require a supermajority of 60 votes to cut off debate and advance to a vote. When a Senate majority fails to get 60 votes to end debate, we typically say that a measure has been filibustered.

But senators can’t filibuster budget resolutions or reconciliation because the CBA limits debate on budget resolutions to 50 hours and reconciliation bills to 20 hours. The limit doesn’t include time spent considering amendments. That means that after debate time expires, senators typically engage in a “vote-a-rama” — often voting on amendments into the wee hours of the night and ending only with the consent of all 100 senators. The Senate then moves to a final simple majority vote on the measure.

What’s the path ahead?

The House and Senate have begun considering the budget resolution. This blueprint includes “reconciliation instructions” — the language that officially instructs House and Senate committees to draft portions of the reconciliation bill that will put the budget into law. After the committees act, the House and Senate budget panels bundle up the pieces into a single, omnibus reconciliation bill for consideration (and possible amendment) on the House and Senate floors.

If Democrats do use reconciliation to enact coronavirus relief, lawmakers could begin drafting the reconciliation bill later this month, with the aim of passing and sending it to Biden to sign into law by mid-March.

So what’s the catch?

Already by the 1980s, Sen. Robert Byrd (D-W.Va.) and others believed that majority parties were abusing reconciliation’s filibuster ban to cram it with provisions unrelated to reducing federal deficits. Thus was born the CBA’s Byrd rule to prohibit what the law calls “extraneous” matter — typically policy changes that do not have a direct and substantial impact on the federal budget.

The Byrd rule sets a six-pronged test of what counts as “extraneous.” Applying the law can be tricky. Senators typically try out their arguments for the chamber parliamentarian, a Senate staff member, who gives the bill a so-called “Byrd bath” to scrub provisions she advises violate the Byrd rule.

The presiding officer historically relies on the parliamentarian’s advice to determine Byrd rule violations. Senators can challenge a ruling, but it takes 60 votes to waive the Byrd rule or to overturn a ruling.

That will likely be problematic for Democrats, who must find ways to make nearly $2 trillion in spending and tax matters consistent with the strict limits of the Byrd rule. If the chair rules a provision out of order, Democrats would need to find 10 GOP members to join them to vote to overturn the ruling of the chair — assuming that Democrats remain united.

How might Democrats bend the rules?

Democrats do have some tricks up their sleeves, but only if all 50 members of the Democratic caucus agree to go along.

First, lawmakers can always try to reconfigure elements of Biden’s agenda so they don’t violate the Byrd rule. For example, dictating a higher minimum wage might not pass muster with the Byrd rule. But using the tax code to fine companies that fail to pay the wage might.

Second, a presiding officer typically follows the advice of the parliamentarian, who provides counsel based on relevant precedent. That reduces uncertainty for senators and lessens any perceived partisanship if the chair decided to make those rulings.

But nothing technically binds the chair to heed the parliamentarian’s advice — even if ruling contrary to past precedent would bend, if not break, a durable Senate norm. The presiding officer could rule a Democratic provision kosher under the Byrd rule, thereby shifting the burden to Republicans to muster 60 votes to overturn the ruling.

Ultimately, the Byrd rule is a tough hurdle for a majority seeking to avoid a filibuster. But even tougher might be getting and keeping all 50 senators on the same page.

Correction: An earlier version of this post misstated the amount of Biden’s proposed $1.9 trillion pandemic and economic relief package. We regret the error.