Senate Majority Leader Charles E. Schumer (D-N.Y.) and Minority Leader Mitch McConnell (R-Ky.) have reached an agreement on how to organize the Senate chamber with a power-sharing agreement strongly resembling a deal reached in 2001, the last time the Senate was evenly split between the parties.

But why did Democrats need Republican agreement in the first place, if Democrats’ 50 Senate seats meant that Vice President Harris could break any tie? What was the dust-up about the filibuster, and did McConnell lose — or get what he wanted? And what does this skirmish portend about the next two years for President Biden’s ambitious agenda?

Here’s what you need to know about the deal.

Why does the Senate need a power-sharing agreement?

Unlike the House — which adopts a new set of rules and committee rosters each Congress — the Senate considers itself a “continuing body,” with each new Senate inheriting rules and committee arrangements from the previous Congress until party leaders agree to arrangements for the new Congress.

While talks between Schumer and McConnell remained in stalemate, last year’s Republican committee chairs kept hold of the gavels and new senators had no spots on committees. That left Republicans in charge of whether or when to advance Biden’s Cabinet nominees, even though Democrats held the majority.

The new agreement hands committee reins to Democrats, allocates committee seats and budgets equally between the parties, and empowers Democrats to advance measures to the floor, even if they deadlock in committee.

Why did Democrats need Republicans to sign off on the resolution?

Technically, a power-sharing agreement takes the form of an “organizing resolution,” a measure that allocates committee seats and budgets between the two parties and any other procedural changes sought by leaders.

Here’s the rub: Like other Senate measures (save nominations and a few other exempt measures), organizing resolutions can be filibustered, meaning that 50 Democrats needed at least 10 Republicans to vote with them to end debate on the resolution. The surest route to an agreement runs through the party leaders, which is why Democrats could not just call up their preferred resolution. Without McConnell’s support, Republicans would have filibustered the plan.

So what was the bottleneck?

As the price for agreeing to a deal, McConnell demanded Democrats forswear the “nuclear option” — a set of parliamentary steps that would allow the Democrats to abolish the filibuster by majority vote in this Congress. While Democrats knew they probably lacked the 50 votes necessary to go nuclear, Schumer stood firm and McConnell relented. Some observers say McConnell lost this battle of wills because the power-sharing deal does not rule out Democrats’ ability to change a rule that since 1975 has required 60 votes to end debate on a matter — thereby effectively giving the minority party veto power over bills on which the parties disagree. From that viewpoint, Democrats beat McConnell: They secured a deal to take charge of the committees without giving McConnell a formal agreement to preserve the filibuster.

But the minority leader got much of what he wanted. McConnell likely anticipated that Schumer would refuse to commit: Mounting pressure across the Democratic base and within the Senate Democratic caucus to jettison the filibuster made it unlikely that Schumer and the Democrats would formally make such a promise.

Nevertheless, McConnell was able to divide the Democratic caucus by putting Sens. Joe Manchin III (D-W.Va.) and Kyrsten Sinema (D-Ariz.) on the record in saying they would oppose abolishing the filibuster. In a partisan era with slim majorities, the minority can derail the majority by forcing such intraparty disagreements.

To be sure, Manchin’s and Sinema’s support for the filibuster was widely known. But by getting them to state their objections on the record, and thus spotlighting differences from their party colleagues, McConnell made plain that Democrats today lack the necessary 50 votes to nuke the filibuster. That undermined the current credibility of a Democratic threat to abolish the filibuster. For now, with rare but important exceptions, Democrats will need to bargain with Republicans to enact their party’s ambitious policy goals.

Similar promises, however, have collapsed in the past. For instance, in 2011 both parties promised to preserve the filibuster on nominations and to cooperate generally. Then Republicans blockaded confirmation votes on scores of President Barack Obama’s nominees. Democrats responded by banning most executive and judicial nomination filibusters in 2013. Republicans retaliated in 2017 by abolishing filibusters of Supreme Court nominees.

Here at TMC, we make no predictions about how long Manchin and Sinema will hold out against fellow Democrats should McConnell and Senate Republicans attempt to blockade Biden’s policy goals.