When President Biden took office last week, he promised sweeping, bipartisan legislation to solve the coronavirus pandemic, fix the economy and overhaul immigration.

Just days later, the Senate ground to a halt, with Democrats and Republicans unable to agree on even basic rules for how the evenly divided body should operate.

Meanwhile, key Republicans have quickly signaled discomfort with — or outright dismissal of — the cornerstone of Biden’s early legislative agenda, a $1.9 trillion pandemic relief plan that includes measures such as $1,400 stimulus checks, vaccine distribution funding and a $15 minimum wage.

On top of that, senators are preparing for a wrenching second impeachment trial for former president Donald Trump, set to begin Feb. 9, which could mire all other Senate business and further obliterate any hopes of cross-party cooperation.

Taken together, this gridlock could imperil Biden’s entire early presidency, making it impossible for him to deliver on key promises as he contends with dueling crises.

This reality could force Democrats to choose within a matter of weeks whether they will continue to pursue the sort of bipartisan cooperation that Biden — and many senators of both parties — have preached or whether to pursue procedural shortcuts or rule changes that would sideline the GOP but also are likely to divide their caucus.

“Things move faster and faster nowadays,” said Sen. John Hoeven (R-N.D.), commenting on the rising tensions Friday. “It doesn’t seem like there’s a honeymoon period.”

Much of the current conflict over the Senate rules comes courtesy of veteran Republican Sen. Mitch McConnell (Ky.), who transitioned to minority leader Wednesday after six years as majority leader.

Just hours after Biden’s inauguration, moments after a smiling Sen. Charles E. Schumer (D-N.Y.) was first recognized as majority leader, McConnell pointedly noted on the Senate floor that the country elected a smaller House Democratic majority, an evenly split Senate and a “president who promised unity.”

“The people intentionally entrusted both political sides with significant power to shape our nation’s direction,” he said. “May we work together to honor that trust.”

Two days earlier, he had notified his Republican colleagues in the Senate that he would deliver Schumer a sharp ultimatum: agree to preserve the legislative filibuster, the centerpiece of minority power in the Senate or forget about any semblance of cooperation — starting with an agreement on the chamber’s operating rules.

The calculations for McConnell, according to Republicans, are simple. Not only is preserving the filibuster a matter that Republicans can unify around, it is something that potentially divides Democrats, who are under enormous pressure to discard it to advance their governing agenda.

“Republicans very much appreciate the consistency and the rock-solid fidelity to the norms and rules that make the Senate a moderating force in policymaking,” said Scott Jennings, a former McConnell aide. “The legislative filibuster is the last rule driving bipartisanship in Washington.”

The Senate filibuster has evolved over the course of its history into a de facto supermajority requirement, necessitating 60 votes to end debate and advance legislation. Rarely has one party held enough votes to defeat filibusters without at least some cross-aisle cooperation.

The rule has been eroded over the past decade. After McConnell led a broad blockade of President Barack Obama’s nominees, Democrats under then-Senate Majority Leader Harry M. Reid (D-Nev.) in 2013 allowed executive appointees and lower-court judges to be advanced with a simple majority vote.

McConnell, in turn, eliminated the filibuster for Supreme Court nominees when Democrats threatened to block the nomination of Neil M. Gorsuch in 2017 and two years later changed the rules to more quickly confirm presidential nominees.

McConnell and other Republicans last week reminded Democrats that many of them praised the filibuster in the past — particularly in the two-year period in 2017 and 2018, when the GOP controlled the House, Senate and White House. Twenty-seven Senate Democrats who now serve signed an April 2017 letter calling on Schumer to preserve the status quo.

But most of those Democrats — who watched McConnell exempt Republican nominees from filibuster rules where he saw fit under Trump, after using them to the GOP’s advantage for six years before that to block Obama’s legislation and nominees — now find his early power move to be infuriating.

“We’re not going to go along with it,” said Sen. Mazie Hirono (D-Hawaii), who was among those who signed the 2017 letter. “There will be some kind of resolution that does not involve Mitch McConnell getting what he wants.”

Schumer said as much Friday on the Senate floor, telling McConnell that he considered any guarantee surrounding the filibuster to be an “extraneous demand” departing from the arrangement that the two parties worked out the last time there was a 50-50 Senate, in 2001.

“What’s fair is fair,” Schumer said, noting that McConnell changed Senate rules twice as majority leader. “Leader McConnell’s proposal is unacceptable, and it won’t be accepted.”

While the two leaders later that day cut a deal delaying Trump’s impeachment trial by two weeks — with a nudge from Biden, who wants to see progress on Cabinet confirmations — there is no visible progress on structuring the Senate.

Without an organizing accord, Republicans remain in the majority of most Senate committees — veteran GOP lawmakers such as Sens. Charles E. Grassley (Iowa), Richard C. Shelby (Ala.) and James M. Inhofe (Okla.) continue as chairs of key panels, while veteran Democrats eager to seize the gavels and advance their long dormant agendas can only wait and wonder.

That’s because the old Senate structures — which had Republicans controlling the committees — will remain in place until Schumer and McConnell reach a power-sharing agreement. Newly sworn-in Democratic senators cannot get committee assignments until an organizational deal is struck. (Some committees, such as the Ethics Committee, are already evenly split between the two parties. In other cases, more Republicans than Democrats left the Senate at the end of 2020, giving Democrats the majority by default.) Democrats can’t unilaterally impose an organizing agreement because they would need Republican support to block a filibuster.

Panel budgets and staff hiring also remain frozen pending a deal.

Sen. Richard J. Durbin (D-Ill.), for instance, is in line to be chairman of the Senate Judiciary Committee and thus oversee Biden’s appointments to the Justice Department and federal bench, as well as key legislative items including an immigration and police overhaul. Asked last week about the status of the panel’s chairmanship, he said, “I have no idea.”

Biden’s least controversial Cabinet nominees have moved forward in the first days of his administration, thanks to the unanimous consent of Republicans: Avril Haines was confirmed as director of national intelligence, and retired Gen. Lloyd Austin was confirmed as defense secretary last week, while Janet Yellen is set to be confirmed as treasury secretary Monday. But other, more controversial nominees could remain in limbo while McConnell and Schumer remain at an impasse.

Many senators and aides say the matter can be settled quickly with Schumer acknowledging reality — that many Democrats, including Biden, are not convinced that the filibuster needs to be scrapped.

Biden, who spent 36 years as a senator before becoming vice president in 2009, said in July that he’d “take a look” at filibuster elimination if Republicans bogged his agenda down in the Senate: “It’s going to depend on how obstreperous they become.”

But White House press secretary Jen Psaki indicated Friday that Biden had not yet reached that point, saying he intended to work with both Schumer and McConnell to advance his pandemic relief proposal: “He wants it to be a bipartisan bill.”

Advancing that legislation absent GOP cooperation would not necessarily require changing long-standing Senate rules. Democrats are already eyeing the special budgetary procedure known as reconciliation, which can allow fiscal matters involving taxation and spending to pass with a simple majority vote.

Republicans used it during the Trump administration, for instance, as a vehicle for partisan health-care and tax bills.

But there are nonbudgetary matters that reconciliation simply cannot be used for — including key Democratic agenda items such as climate change legislation, expansions of civil rights and voting access, gun restrictions and more items that have little, if any, GOP buy-in.

That stands to only compound the already immense pressure to ditch the rule — a campaign that is already being pushed by former senators and Senate aides, opinion journalists with considerable influence inside the Democratic caucus and a legion of activists that emerged as a potent force during the Trump administration.

Fix Our Senate, a coalition of liberal and labor groups formed to advocate for filibuster elimination, has already launched a six-figure ad campaign and plans to deploy field operatives in states where Democratic senators have expressed reluctance to ditch the rule.

“There is absolutely no reason to give Senator McConnell months and months to prove what we absolutely know — that he is going to continue his gridlock and dysfunction from the minority,” said Eli Zupnick, a spokesman for the group.

The pressure is also coming from within the Democratic caucus, where key voices are urging Schumer not to let Republicans weaponize Senate rules — even as McConnell threatens to paint them as hypocrites for abandoning their pledges of bipartisanship.

“Millions of people are giving up on their government because they’re hurting, and we are not responding,” said Sen. Bernie Sanders (I-Vt.), who caucuses with Democrats and will have a key role in the reconciliation process as the incoming Senate Budget Committee chairman. “We have an enormous agenda, and we have got to move as quickly as we can, and in my view, we’ve got to use all of the tools that are available.”

The path ahead is likely to be decided by a small group of moderate Democrats, elected from red and purple states, who have signaled support for keeping the filibuster while hinting that their patience for partisan obstruction might not be infinite.

Sen. Joe Manchin III (D-W.Va.) has been the most outspoken Democratic opponent of changing Senate rules and has sought to assemble a bipartisan cadre of centrist senators willing to hammer out deals across the aisle. But other Democrats are similarly resistant. A spokeswoman for Sen. Kyrsten Sinema (Ariz.) said the senator is “against eliminating the filibuster, and she is not open to changing her mind about eliminating the filibuster.”

Meanwhile, other Democratic senators, including Jon Tester (Mont.), have also signaled support for the status quo while hinting that GOP stonewalling could change their minds.

Manchin told reporters last week that while his mind hadn’t changed on preserving the filibuster, he backed Schumer as he seeks to hammer out an operating accord with McConnell. And he signaled that, when it comes to a dysfunctional Senate, there is one Democrat he may take his cues from going forward.

“If there’s one person who can make it work, it’s Joe Biden,” he said, adding that the president “understands how this place used to work, how it should work and how it can work — if it doesn’t work under Joe Biden, it doesn’t work at all.”

Correction: An earlier version of this article mistakenly reported that Sen. Kyrsten Sinema (D-Ariz.) has suggested she might be willing to eliminate the filibuster. A spokeswoman for Sinema said the senator is firmly opposed to doing so and is “not open to changing her mind.”